I have a great little book called "Calendar" by David Ewing Duncan, it explains the history of the Calendar, how it developed etc and it does have an explanation about the differing dates of Easter. It has been awhile since I read it, but it had something to do with the change from the Julian (?) to the Gregorian calendar. I think Orthodox Christians chose not to follow the new calendar
I'm going to write this down. I wonder if our minister has it? It'd be the kind of thing he would have if he knew about it. Thanks Astrid!