Assuming that all sets are being performed to failure, I think that each rep increase (on any set) will reflect a very small increase in muscle fiber size in advanced lifters performing single-joint exercises. There might also be a small increase in motor unit recruitment of the agonist muscle, in addition to an improvement in coordination.

Periodization of load isn’t really that interesting for hypertrophy when using single-joint exercises. It can play a role in multi-joint exercises, because it alters the contribution of each of the prime movers to the overall force production in the exercise. In this way, it plays a similar role to exercise periodization. Obviously, it can be helpful in other ways, however, as you suggest.