Here's my analysis:
There are twelve people in the picture. It's cut into two halves, top and bottom, effectively cutting each of the men into two (unequal) pieces. By shifting the top pieces, the illusion is created that
all of the pieces are recombined.
But that isn't the case. The leftmost man never gets a replacement for his scalp and the third man from the right parts with his soles, and ends up somewhere in the middle, still without soles. Because two men out of twelve are not recombined, you end up with eleven men and two pieces. These pieces are large enough to be mistaken for complete men, but they are not.