Here's what I know about this from a professor at a top astro school. I assume other schools are similar. The first round consists purely of numerical rankings. The best people they get by numbers are immediately offered admissions. These are the people with 990 gre's, 4.0 gpas, a few publications and the like. Then they actually dig in to the applications and find the best ones. However, remember that this takes time. Each of the committee members spends about 1/2 hour on each application. Some schools filter (the ones that get hundreds) but many will read every application. The school I know about gets about 100 astro applications and they read every one. Its time consuming, and this is why there are two waves.