I read up on all of this on an science essay written by the late Isaac Asimov - one of the greatest scientific authors of all time.
tl:dr version:
Certain ideas were in place, but they were found to be inaccurate - so scientists began to sort out the kinks.
Everything worked well, and the Pope kept Easter on the Lunar calendar, hence why it changes all the fucking time.
It was fine until a scientist realised that they were still inaccurate by a tiny amount - so the pope made the world change their calendars back a few months/years/whatever to fix it, and introduced leap years to fix it.
Side note:
Aman sorted out the whole "AD and BC" system...
BUT GOT IT WRONG as things were worked out again - 6 years incorrect in fact.
Hence, Jesus is listed as being born in 6 B.C.
Weird, huh?