Because XL displays numbers according to the number format of the cell.
If you have a fixed number of digits you want to display, choose
Format/Cells/Number/Custom and enter
00000
for example, if you want 5 digits.
Alternatively, preformat the cell as Text or precede the value with an
apostrophe ('). This will convert the number to text, so while math
operators will coerce the text back to numbers, SUM() will ignore them.
