This is a slightly complicated topic. It comes down to a substantial difference between elite state institutions and elite privates, undergraduate and graduate programs, and engineering/cs vs general majors.
Elite private universities (stanford, harvard) tend to have very small undergraduate populations and draw from a national or international pool of applicants. This of course makes them highly selective, as spots are scarce and the applicant pool is large. Public universities (Berkeley, UW) have very large undergraduate student bodies (often 5 times the size of a small private) and require a much larger portion of in-state students. More slots, smaller applicant pool means a much higher admission rate and generally lower numbers (SATs and so forth). Like another poster mentioned, 25%ile at MIT > 75% at UW, but the top 25% of the class at UW is larger than the entire undergrad population at MIT. The difference in nature of undergraduate admissions makes the averaging kind of meaningless.
At the graduate level, elite private and public universities tend to admit roughly similar numbers of students. And not surprisingly, admission rates are much more comparable. Elite publics, for some reason, seem to do particularly well at the grad level in engineering and computer science (aside from Cornell, the ivies don't really show up much on the top 10 lists, while Berkeley, Illinois, Michigan, Texas, and so forth are highly ranked - by the same magazine (US News) that doesn't put a single public into the top 20 at the undergrad level).
Lastly, there is the question of the degree itself. At top publics, engineering often requires a second admissions process - you get into Berkeley or UW, and then you have to keep your grades high enough to gain admission to the engineering or CS major. You can gain admission directly to the major from high school, but that's tougher. And the process of getting the degree itself is daunting - I don't know much about UW, but the coursework in CS at Berkeley is extremely rigorous (and I have no reason to think it wouldn't be at UW as well). As with UW, Berkeley does have a much larger undergraduate population. It's less selective because of that (and the in-state quota), but on the way out, we're talking about students in the top quarter of the class, from a major that is extremely difficult to enter and complete.
All in all, I'm not surprised that people would feel that the undergrads who make it out of CS from UW are among the top grads that year. The grad level programs are already known to be top 10, so I don't think that even comes up.