Face going away. (hidden surface removal)

If I make a box....

Then make a smaller box to fit inside that box....

Then use the CSG Subtract tool to hollow the box out, I will get seven brushes.

top, bottom, left, right, front back, AND the original box I used to hollow it out...

Still with me?

Now, I take that inside box (original smaller box that fit inside the larger box) and discard it...

I am left with six brushes...

If I do this, one of the bruhes left always has a hidden face. It's driving me nuts!

What gives?

Thanks ahead of time.