Forget about the small difference in percentages in some counties, it is well within the margin of error. It is certainly a mix of all these factors. Better schools is definitely a very important factor and those schools are more expensive (if private) or in the area with higher property value (if public). So better schools and higher household income go hand-in-hand. Clearly, the two counties with the highest median income have several times higher % undergrad population than the % population.
Perhaps I misunderstand your “geographic factor” which usually refers to location relative proximity. My first thought was Jackson county which is closer than some of the 3 counties you listed (excluding Washtenaw where UMich is located) but does not seem to be a feeder county.