I noticed something similar years ago (long before starting strength), although at much lower volumes than you were doing. I started doing 4 sets across on pullups, and adding a rep when I could do all the reps of all 4 sets. If I did not get all the reps on the last set then I would do some negatives or something. I made better progress this way than any other regimen I have tried. I liked that it was mentally easier than some other approaches, because the first two sets were fairly easy and only on the last set was I really pushing myself to failure.
Maybe what Rip is hinting at is that the sets-across approach is the same as the SS 3x5 program, except the parameter that is changed is reps, not weight. We don't vary the weight on each set so that each is a maximal effort. In each case there is are a number of sub-maximal sets and then one set more or less to failure. That, combined that with what I think is the commonly-agreed-upon fact that pullups respond well to volume and frequency, could explain your success.