If the cell format is General, you can have max 11 digits in the cell and if the digits are more than 11, the scientific notations will be displayed in the cell whereas the actual value underneath the cell remains intact and you can check it if you look at the formula bar.
To display the number correctly if the digits are more than 11, change the cell format to Number with zero decimals.
Btw if you only have to enter a large number in a cell i.e. a number with more than 15 digits, you may format the cell as Text and you can enter a number with more than 15 digits. But in your calculations, the 15 digits limit comes into play.