subtraction returning incorrect calue

We have a spreadsheet that subtracts one number from another. We did not
understand why Excel is rounding the way it did so we decided to add decimal
places until we see the value change. For instance, one of our number was
going out to x,xxx.499999999999999999999999999999900000 while the other
number was x,xxx.5000000000000000000000000000000000. When we rounded the
numbers, one rounded up while the other rounded down. This is fine. Now
that we fixed the formula to TRUNC instead of ROUND, I thought the issue
would go away but now when I subtracted two numbers that were like
x,xxx.xx000000000000000000000000 and another number that is also
x,xxx.xx000000000000000000000000, why would I receive an answer that comes
back as x,xxx.xx4999999999900000000000000000? This doesn't make sense to me.
Has anyone else run into this issue? How did you fix it?

--
Ashish