Isn't the purpose of dinner to "feed" those who stay late knowing there is food and know that they won't have to go hunting outside to see what joint is still open (specially in the suburban campuses) rather than being a venue to feed staff on their way out (home, etc.)?
From that PoV 7-ish makes sense. I know I would not stay around till 7 just for a freebie dinner if I didn't have anything to do.