It's called summer melt. Students will continue to change their minds for various reasons up until move in day (they get off another wait list, they were double deposited, etc.). Last year when the class of 2015 was going through orientation and dorms had just been assigned the class was about 100 students over what they had anticipated, thus forcing triples. I don't know for sure, but I believe but the time the official census was taken in October that number had dropped considerably.
The reason why a class can be 'over' while still taking students off the wait list is because you are dealing with several distinct groups of students, not just one (10 I believe...in state and OOS for SEAS, CLAS, NURS, ARCH, & Kinesiology). You may find after May 1 you have room in the class for OOS Architecture students and can make some wait list offers there, despite being over enrolled in In state Arts & Sciences.