Delay in calculating last field subtracted with simple field notation

I have a form that is pretty much a grid of numbers. There are "totals" rows and columns, and I am using simple field notation to add up these totals. A couple of these "totals" rows are then subtracted from another row.

The problem I am having is that for some reason the last form field entered does not seem to apply to the calculated total immediately after tabbing or clicking out of the field. However, next time I enter a number, the calculation happens and everything is correct.

This only seems to happen with fields where subtraction is involved, the fields that are just added together are fine.

There are no "dots" in the field names, and the calculated fields are all read-only.

Another strange happening is that I get a warning sound from Windows (the one that usually happens when I try to do something that can't be done) every time I enter a value in a field. However the warning sound goes away when I enable Reader editing capabilities and fill out the form in Reader. Rather strange, I'd love to hear an explanation if anyone has one!

I am using Acrobat 8.1.2 Pro on Windows XP.

Thanks