It's a matter of processing time, in some cases. We try to have as many countries at 7:00 local as possible, but in Europe, for example, we need to spread them out to avoid server strain.
For the "wider" countries (USA, Russia, Canada, etc.) we try to make the game as accessible as possible for all time zones in the country.
For the USA, we also need to take into account (though they're small) Hawaii and Alaska :-D
Edited 9/24/2007 7:03:21 PM by BB-Mark