Обьяснить кусок кода, почему он работает хотя по идее не должен

window.onload = function() { 
  var numbers = document.querySelectorAll('.btn-number'), 
    operations = document.querySelectorAll('.btn-operation'), 
    clears = document.querySelectorAll('.btn-clear'), 
    display = document.getElementById('display'), 
    decimal = document.getElementById('btn-decimal'), 
    MemoryCurrentNumber = 0, 
    MemoryNewNumber = false, 
    MemoryPandingOperation = ''; 
 
  function searchCurrentElem(variable, funName) { 
    for (var i = 0; i < variable.length; i++) { 
      var CurrentElem = variable[i]; 
      CurrentElem.addEventListener('click', function(e) { 
        funName(e.target.textContent); 
      }); 
    }; 
  }; 
  searchCurrentElem(numbers, numberPress); 
  searchCurrentElem(clears, clearPress); 
  searchCurrentElem(operations, operationPress); 
  decimal.addEventListener('click', decimalPress); 
 
  function decimalPress(cell) { 
    var localDecimalMemory = display.value; 
    if (MemoryNewNumber) { 
      localDecimalMemory = '0.'; 
      MemoryNewNumber = false; 
    } else { 
      if (localDecimalMemory.indexOf('.') === -1) { 
        localDecimalMemory += '.'; 
      }; 
      display.value = localDecimalMemory; 
    }; 
  }; 
 
  function numberPress(symbol) { 
    if (MemoryNewNumber) { 
      display.value = symbol; 
      MemoryNewNumber = false; 
    } else { 
      if (display.value === '0') { 
        display.value = symbol; 
      } else { 
        display.value += symbol; 
      }; 
    }; 
  }; 
 
  function operationPress(op) { 
    var localOperationMemory = display.value; 
    if (MemoryNewNumber && MemoryPandingOperation !== '=') { 
      display.value = MemoryCurrentNumber; 
    } else { 
      MemoryNewNumber = true; 
      if (MemoryPandingOperation === '+') { 
        MemoryCurrentNumber += parseFloat(localOperationMemory); 
      } else if (MemoryPandingOperation === '-') { 
        MemoryCurrentNumber -= parseFloat(localOperationMemory); 
      } else if (MemoryPandingOperation === '/') { 
        MemoryCurrentNumber /= parseFloat(localOperationMemory); 
      } else if (MemoryPandingOperation === '*') { 
        MemoryCurrentNumber *= parseFloat(localOperationMemory); 
      } else { 
        MemoryCurrentNumber = parseFloat(localOperationMemory); 
      }; 
      display.value = MemoryCurrentNumber; 
      MemoryPandingOperation = op; 
    }; 
  }; 
 
  function clearPress(btnClear) { 
    if (btnClear == 'ce') { 
      display.value = '0'; 
    } else if (btnClear == 'c') { 
      display.value = '0'; 
      MemoryCurrentNumber = 0; 
      MemoryPandingOperation = ''; 
    }; 
  }; 
 
 
};

#calc { 
  position: absolute; 
  top: 50%; 
  left: 50%; 
  transform: translate(-50%, -50%); 
  width: 500px; 
  height: 700px; 
  background-color: pink; 
  border-radius: 20px; 
  padding: 20px; 
  display: grid; 
  grid-template-rows: 0.7fr 1fr; 
  grid-row-gap: 20px; 
} 
 
#display { 
  width: 100%; 
  height: 100%; 
  border: none; 
  outline: none; 
  background-color: white; 
  color: black; 
  border-top-left-radius: 10px; 
  border-top-right-radius: 10px; 
  font-size: 70px; 
  display: flex; 
  text-align: right; 
  padding-left: 20px; 
  padding-right: 20px; 
} 
 
#calc_display { 
  width: 100%; 
  height: 100%; 
  display: flex; 
  justify-content: center; 
  align-items: center; 
} 
 
#calc__clock-face { 
  display: inherit; 
  grid-template-columns: 1fr 1fr 1fr 1fr; 
  grid-template-rows: 1fr 1fr 1fr 1fr 1fr; 
  grid-column-gap: 5px; 
  grid-row-gap: 5px; 
} 
 
.calc__btn { 
  border: none; 
  outline: none; 
  background-color: #fff; 
  color: #000; 
  font-size: 25px; 
  font-weight: 500; 
  display: flex; 
  justify-content: center; 
  align-items: center; 
  text-align: center; 
  text-transform: uppercase; 
  transition: 0.3s; 
} 
 
.btn-operation:hover { 
  background-color: skyblue; 
} 
 
.btn-clear:hover { 
  background-color: goldenrod; 
} 
 
.btn-number:hover { 
  background-color: darkgrey; 
} 
 
#btn-decimal:hover { 
  background-color: greenyellow; 
} 
 
.calc__btn:nth-of-type(10) { 
  grid-row-start: 5; 
  grid-row-end: 6; 
  grid-column-start: 1; 
  grid-column-end: 3; 
  border-bottom-left-radius: 10px; 
} 
 
.calc__btn:nth-of-type(11) { 
  grid-row-start: 1; 
  grid-row-end: 2; 
  grid-column-start: 1; 
  grid-column-end: 2; 
} 
 
.calc__btn:nth-of-type(12) { 
  grid-row-start: 1; 
  grid-row-end: 2; 
  grid-column-start: 2; 
  grid-column-end: 3; 
} 
 
.calc__btn:nth-of-type(13) { 
  grid-row-start: 1; 
  grid-row-end: 2; 
  grid-column-start: 4; 
  grid-column-end: 5; 
} 
 
.calc__btn:nth-of-type(14) { 
  grid-row-start: 1; 
  grid-row-end: 2; 
  grid-column-start: 3; 
  grid-column-end: 4; 
} 
 
.calc__btn:nth-of-type(15) { 
  grid-row-start: 2; 
  grid-row-end: 3; 
  grid-column-start: 4; 
  grid-column-end: 5; 
} 
 
.calc__btn:nth-of-type(16) { 
  grid-row-start: 3; 
  grid-row-end: 4; 
  grid-column-start: 4; 
  grid-column-end: 5; 
} 
 
.calc__btn:nth-of-type(17) { 
  grid-row-start: 4; 
  grid-row-end: 6; 
  grid-column-start: 4; 
  grid-column-end: 5; 
  border-bottom-right-radius: 10px; 
} 
 
.calc__btn:nth-of-type(17):hover { 
  background-color: red; 
}

<div id="calc"> 
  <div id="calc_display"> 
    <input id="display" type="text" name="display" placeholder="0000000000" disabled value="0"> 
  </div> 
  <div id="calc__clock-face"> 
    <button class="calc__btn btn-number">1</button> 
    <button class="calc__btn btn btn-number">2</button> 
    <button class="calc__btn btn-number">3</button> 
    <button class="calc__btn btn-number">4</button> 
    <button class="calc__btn btn-number">5</button> 
    <button class="calc__btn btn-number">6</button> 
    <button class="calc__btn btn-number">7</button> 
    <button class="calc__btn btn-number">8</button> 
    <button class="calc__btn btn-number">9</button> 
    <button class="calc__btn btn-number">0</button> 
    <button class="calc__btn btn-clear" id="ce">ce</button> 
    <button class="calc__btn btn-clear" id="c">c</button> 
    <button class="calc__btn btn-operation">*</button> 
    <button class="calc__btn btn-operation">/</button> 
    <button class="calc__btn btn-operation">+</button> 
    <button class="calc__btn btn-operation">-</button> 
    <button class="calc__btn btn-operation">=</button> 
    <button class="calc__btn" id="btn-decimal">.</button> 
  </div> 
</div>