Why does Excel 2010 automatically reformat numbers?

You don't have to change the cells' formatting one at a time. You can apply the same formats to whole columns, ranges or even sheets. If you know that you'll have number in all the cells, make sure you set them to one of the number formats. The General formatting option is treated by Excel as a sort of "no formatting" option, so it will often try to guess what specific format you want it in.

Alternatively, you may want to look at this answer and see if it applies to your situation:

Excel assumes you have not applied a format if you use general, so it tries to find the best format for the data you entered. To disable this, go to File => Options => Advanced and uncheck the option for "Extend data range formats and formulas."

I had a similar issue.

I had numbers formatted as percentages with one or two decimal points. Whenever I changed the cell value by adding/subtracting a % (e.g. the formula is =A2-.2%), it reformatted the cell with 15 significant digits. Ugh!

I tried the tips above (e.g. deselecting Extend data range formats and formulas) and they didn't help. I tried re-formatting the cells as General, then applying the % format, and then updating the formula. Same problem. I went into custom formats and deleted the % format with 15 decimal points, but as soon as I re-edited the cell's formula, Excel magically re-created the 15 digit custom format and applied it.

I finally found a solution:

When adding/subtracting, don't use the % in the formula. Instead of =A2-.2%, use =A2-.002.