Are most people moving in on the first day of their contract? That might sound daft, but some (if not all, I'm not really sure) contracts start on the 15th September, which is a Wednesday, making it a bit inconvenient for people travelling from far away. Are lots of people moving in at the weekend instead?
Personally, I would like to move in ASAP, but if the majority of people are waiting until Saturday... the thought of sitting in an empty flat for 3 days is somewhat unappealing!