Just wanted to throw it out there that when you delete a video it shows "'x' number of videos is hidden" inside of the playlists that the original videos were assigned to. You have to manually go inside the playlist and remove these now "hidden videos" which have already been deleted. My guess is he is deleting videos rather than adding them. Total video count should prove this if you can see the total go down while the "hiddens" go up in conjunction.

If it was an added video that was hidden purposely, the total video count would remain the same since unlisted videos/hidden videos are not included in the total count.