formulas producing unjustified results
 

using "if" functions such as"=IF(SUM(E27:M27)-D27=0,"",SUM(E27:M27)-D27)" I
keep getting unexpected results such as -9.09495E-13 when formula code is set
to general or -0.00 when it is set to two decimal places. No cells within the
range have more than two decimal places and there are no hidden cells
involved either. This is not happening everytime, maybe once or twice on a
spreadsheet where the formula is applied up to 30 times. Can anyone explain
why this is happening and how to stop it in future please?