Question

Its not that unusual for the reception to be later than the ceremony, esp if its a different location.
The bride and groom might not have been able to get their recpetioin choice any earlier than seven pm. They could be wanting to include people that couldn`t make it to the ceremony because of work or whatnot that could come to a later reception.
There are lots of reasons that they might have scheduled the reception later. Heck some people on this board are having the reception on a different month altogether

The guests are to do whatever they want between the ceremony and the reception - go home, go site seeing, etc. You aren`t expected to remain in your dress clothes you wore to the wedding all day. You should inquire as to what they would like you to wear to the reception though in case they are having a formal evening reception.

Also you should find out if dinner is being served at the reception or if you are expected to have had dinner before you show up.

Thanks!