Format doesn't change the value stored in the cell (unless you have the
Precision as displayed option set).

See http://cpearson.com/excel/rounding.htm for an explanation of
small rounding errors inherent in performing finite precision math.

In article ,
Gator wrote:

Why is there a number being calculated when I am subtracting the same
amounts? I have a sum in one cell and a number in another - which are
essentially the same amount but Excel is showing a decimal number like
.0000000021. Also, I am formating the cell as number with two decimals - the
default, I believe, when the comma "," icon is selected in the toolbar.
Also, the number shows 0, but when I extend the decimal places I see that it
is the number above. Also, I have a similar situation in another record but
it shows the "-" symbol for zero.