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How do you explain this, though? Is it due to a large portion of this sample being lower ranked privates? (For the record, not agreeing with OP.. just curious.)
http://www.gradeinflation.com/ (~3.3 for privates, 3.01 for publics)
Isn't another obvious answer that the caliber of average students from the aggregate private vs. public schools are just higher? If you actually look at the sample schools - the private schools included are more or less at the top of the rankings while the public schools have many that are not as well known (yes they do include the top public schools such as UMich, Berkeley, UCLA). However, with large public universities, there are bound to be a disproportionately larger number of students who drag down the GPA average when compared to top private schools. While I do think that there are many highly intelligent and competent people at state universities, it is a given that the % of these type of students among the entire student body is much lower.

Undergrad is what you make of it. I didn't need competition or some hard-ass teacher breathing down my neck with the threat of bad grades to make me learn the material well. I just happened to have the added bonus of a significantly lower level of stress associated with my learning experience. I learned it well enough, despite not going to some top 30 or whatever, as evidenced by my 35 on the MCAT, so clearly learning and comprehension outcomes aren't directly correlated to what school you attend.