Rather than have us guess, would you please tell us what value is in A before the copy operation AND what value is in B after the copy? And what you EXPECTED to be there?
I know this is a "committee" account. You folks have asked many questions and we are happy to help, but it would be nice if you remember from one question to the next - and share this fact with the others in the committee - that we are NOT mind readers and like to have at least SOME hint of what is wrong and why you THINK it is wrong. Saying "correct our rounding error" points to a possible cause, but it would make it easier to confirm suspicions by knowing what you expected and what you got.
The underlying data elements appear to be integers, but when the form opens, one of those elements shows values that cannot be of type integer. This MAY be an error with the underlying data type for that bound field in that form.