I think what he's saying, is this:
No bands, I can get 80 for 6 and 85 for 4.
With bands, I can get 70 for 6 and 75 for 2.
Yes, I am making up random numbers. Just trying to translate here. So he is saying, why do his max reps decrease; that is, the decline on the graph of the ratio of weight:reps with bands is steeper than the decline on the ratio of weight:reps without bands.
I'd wager it's because bands are harder on you than straight weight. You have to fight the pull, getting set up is harder, there's probably more CNS involvement, etc.
But I'm just an internet expert.