TJR, it's the angles of the cuts, and the slop coming out. The cuts are not parallel to each other throughout, leading to the "squares" not being truly full "squares" throughout.
Look, for example, at 2:34. Look at the fourth and fifth rows from the bottom. Does that row of squares look complete, or normal? No.
Note the video cut at :36. I can assure you, he does NOT place the pieces down on the mat in the same orientation as they came out of the box. In fact, I'm pretty certain they're not even the same pieces. Same goes for the video cut at 2:21. Look at :49, when he first assembles the pieces outside of the box, how the diagonal line through the middle of the pieces is so sloppy, and how some of the horizontal lines don't line up at all? Not coincidence. Even as he does his sliding, the lines still continue to not line up. It's those irregularities that result in the "extra" pieces.