Access 2K2: making numbers up in a query sum

Hi all!

I am running a query in a subform where I sum a [number of units] *
[operation sign]. Units are imported from a csv file with 3 decimals and
operation sign is either -1 or +1:
For example, [440.000*-1] + [550.070*+1]... etc...

For a strange reason, Access shows 7 to 9 decimals in the sum's result. Now
they are not zeros and it seems like it's calculating on something but I
can't figure what!
The problem is that the result is wrong, and the only way I can get a good
result is by formating like "#,##0.000" the product of the two.

This is just insane and I can't understand why it would do that...

Any thougts?
Thx
T_D