Why there is a difference in Easter date calculations of Eastern and Western Churches?
Between 326 AD and 1582 AD Easter Sunday dates were based on the Julian calendar in use at that time. Pascha Sunday was defined as the Sunday following the Paschal Full Moon (First Full Moon after the vernal equinox) date for the year, using a simple “19 Paschal Full Moon dates” table of the “Old” Julian calendar. This was replaced in the Western Church in 1583 AD by a new larger (revised) “Paschal Full Moon Gregorian dates” table. The Eastern Church continues to use the 19 Paschal Full Moon dates table.
So the current Easter calculations of Eastern and Western Churches use differing cycles of Ecclesiastical Full Moon dates and different tables, so we should not be surprised that the results frequently turn out differently.
The Orthodox Church uses the fixed date of 21 March in the Julian calendar in order to calculate the vernal equinox, while the Western church uses the fixed date of 21 March in the Gregorian calendar (The 21st of March in the “Old” Julian calendar is the equivalent of the 3rd of April in the Gregorian calendar until 2099, and then it becomes the 4th April from 2100).
In some years the Orthodox Easter Sunday does occur on the same day as the Western Easter Sunday. For example, this occurred in 2004 because the Western Easter Sunday date of April 11th, 2004 (Gregorian calendar) was the same day as the Orthodox Easter Sunday date of March 29th, 2004 (“Old” Julian calendar). Year by year, Orthodox Pascha moves back and forth between its earliest date on April 4th and its latest date on May 8th - as set by the rule of the Council of Nicaea in 325AD using the “Old” Julian Calendar. The Western Easter also moves back and forth, between its earliest date of March 22nd and its latest date of April 25th, but the two movements are not always together. There is are sometimes an unusually wide gap when the Western Easter falls at an early date in its cycle, while the Orthodox date falls late in its cycle.