I'm not referring to just CS101; I'm talking about the whole curriculum. That bimodality tends to go away once you're done taking classes that are a re-hash of what the experienced kids already know as freshmen.

And no, it's not just the effect of "weeding out" the inexperienced kids -- as I said above, a number of the best students in my program had no prior experience, and a fair number of "experienced" kids flamed out once they hit the non-coding portion of the curriculum.

I went to a small school for undergrad, so I knew everyone who graduated with me, and everyone who started with me. Later, I went to a very large graduate school, and while the same patterns seemed to hold, I can't say for sure.