Because he was not doing the same number of reps for both sides in the one hand exercises!!.. starting from minute 8:26 he did 8 reps for his left arm (including 2 forced reps) and then 9 reps for his right (including only one forced rep)!!..
this was of course to tease dorian's boys

.. but frankly speaking blood & guts is a good training video and for those who didnt see before it's uploaded now in 6 parts on youtube..