I have 75 kids in three sections, and I'd have 100 kids in four sections if they'd open up a fourth section. And all of my students except maybe the bottom 5-10% are moving confidently through the material at their own pace.
I'll bet your school either has 1) a lot fewer students 2) a much more hand-picked set of kids, or 3) is leaving a much higher percentage of kids behind, who are either lost or cheating.
My school is a small (~130 per grade), but renowned international school in Shanghai, China. Your first two guesses are very much spot-on, but I'd have to disagree with the third. Those of us that are ahead help out with the one's who are struggling and in the end, I'd like to think that it all works out well.
With that in mind, however, your numbers are indeed more impressive. Because our class size is so small, we're constantly involved in face-to-face interaction with our instructor and with our peers. I doubt our pace would translate into the same success in your school. Is your course designed for the AP exam?