Are you sure your example is right?
If the column width is too narrow, excel will only display the rounded
number--but in this case, it would be 12. (If you typed 12.73, then excel may
display 13.)
Try widening the column a bit (or changing to a smaller font)
If that's not it, what do you see in the formula bar? If you really see 13,
then I would suspect that you have an event macro that's running and rounding
(up) that entry.
You can find out by opening up your workbook and not allowing macros to
run--then see what happens when you test it by typing in a new value.