High admission rate may also means less applicants. UIUC really sucks in financial aid. Also, you apply directly to a major. So most applicants are quite determined and they are well aware of the mid 50 for admission. Although there are difference in admission stat among different colleges within UIUC, the admission rate for College of Engineering there was still 62% in 2013 which is not far off from the overall admission rate.
http://profiles.asee.org/profiles/6085/screen/19?school_name=University+of+Illinois+at+Urbana-Champaign