I showed this to my daughter and asked her to figure it out for me. She cut it out in graph paper, cut the pieces, and then showed me how when she went to reassemble them, they didn't truly fit right, despite what the video clip shows. That accounted for the extra square. She said the slopes of the boxes didn't match and that's why they didn't fit together right.

I wouldn't have thought to do that...

I don't think I am smart enough to understand that.Oh well.

It looks similar to the way one quilt teacher ( John Flynn) was showing how to make a quilt backing by cutting the fabric at an angle and sliding it.

Maybe this explains why our quilts don't always fit together like they should!