If you will format the two cells to display 11 decimal places, you will find
that at least one of the values is not 2458.69.
Assuming that these cell values are the result of simple additions and
subtractions of 2-decimal place numbers, then the obvious question is how you
got junk in the 11th decimal place. That gets you into the realm of Bob
Phillip's answer.
Excel (and almost all other general purpose software) does binary math.
Most terminating decimal fractions are non-terminating binary fractions that
can only be approximated (much as 1/3 can only be approximated in decimal).
In fact, the only 2-decimal place numbers that can be exactly represented in
binary are .00, .25, .50, and .75.
Initially you cannot see the impact of these approximations because Excel
(as documented) will display no more than 15 significant figures. But after
several arithmetic operations (or after taking the difference of two numbers
that are equal in the most significant figures) the impact of the
approximations can accumulate to detectable levels.
I have never seen an example where Excel performed basic arithmetic
incorrectly; and so in the absence of specifics for your case, I must presume
that it again is doing the math correctly per the IEEE 754 standard. If (as
I suspect) the issue is simply accumulation of approximations to numbers that
have no exact binary representation, the obvious solution is to eliminate
them by appropriate rounding. MS cannot do that for you automatically,
because "appropriate" depends on the specific calculations that you are
performing. If (as I assumed previously) you are merely adding and
subtracting numbers with no more than 2 decimal places, then you could round
the result to 2 decimal places without violence to the calculations.
If you want to see the actual values of the approximations, the code at
http://groups.google.com/group/microsoft.public.excel/msg/b106871cf92f8465
will help.
Jerry
