I think your suggestion about graduates from top schools being above the 90th percentile is quite plausible.

Let's see. This article from 2013 reports that 56,742 students graduated with CS majors in 2012. It seems that there are on the order of 100 CS graduates at each top school per year (with the number increasing rapidly over time), so maybe 1,000 total, so 2% of CS graduates. So yes, it's plausible that they're above the 90th percentile.

Also, part of it could be that programmer salaries have gone up recently, so that the mid-career data you cite won't be in line with the current starting salaries. This would match up with the anecdata I have that starting salaries for college hires at Microsoft, Amazon, etc. are about 30% higher now than 7 years ago. Have salaries for other engineering jobs increased at the same rate?

The 2008 source that I cited in particular is dated.

Is it plausible that starting salaries would increase faster than mid-career salaries? Why wouldn't salaries for senior software engineers rise by 30%, too?