Julian/Gregorian calendar is only one part of the reason (that accounts for the "big" difference (0 or 1) vs (4 or 5)). The other part (the reason why there is a "small" 1 week difference) is due to the different methods in calculating the lunar phase.

This year we get both combined, so there is a massive 5 week difference between the two.

The Western Easter is more "astronomically" accurate, but the Eastern Easter always takes place after Passover, so it's more "religiously" accurate.

I have parents from both backgrounds so we celebrate both Easters. Unless of course they fall together anyway.

EDIT:
As a side notice, some Christians up till recently (not sure if still) had yet another method of calculating Easter. Basically, while most Christians celebrate it on Sunday, the same day of the week Jesus was resurrected, they'd instead celebrate it on the same day of the month Jesus was resurrected, as per to the Hebrew calendar, which is a day or 2 after Passover, not necessarily on a Sunday. Not sure who exactly does/did this, but I vaguely recall it being practiced by a few communities somewhere in Georgia/Caucasia. I could be mistaken though.