Why does Orthodox Easter often fall on a different date than Catholic Easter?

The Orthodox nuance

Before Nicaea, Christianity was divided on this issue: some communities in Asia Minor, called Quartodecimans, celebrated Easter exactly on the fourteenth day of the Jewish month of Nisan, whatever day of the week it fell on, following directly the calendar of the Jewish Passover. Others insisted that the Christian feast always fall on a Sunday, the day of the Resurrection. The Council of Nicaea chose the Sunday rule, precisely so there would be a single date for the whole Church, not to divide, but to unite.

For a long time this rule worked well, because the whole Christian world used the same calendar, the Julian one, named after Julius Caesar. Except the Julian calendar year is slightly longer than the real solar year, by about eleven minutes, a small gap that nonetheless accumulates over the centuries, so that by 1582 the Julian calendar had fallen ten days behind the real course of the sun. Pope Gregory XIII corrected this through a reform, the Gregorian calendar, gradually adopted by the West and then by much of today's secular world.

The Orthodox Churches kept the calculation of Easter according to the old Julian calendar, even where, as with the Romanians, the corrected calendar was adopted for fixed-date feasts — which is why Christmas falls on December 25 just as for Catholics, but Easter remains calculated separately, by the old rule, and often does not coincide. There is also an older principle, not explicitly formulated at Nicaea but supported by many Orthodox canonists through the centuries, that Christian Easter should not fall before or on the same day as Jewish Passover, so that the historical order remains clear: first the Passion, which took place around the time of the Jewish Passover, then the Resurrection. The exact practice, however, has varied over time, and it must honestly be said that not all Church historians agree on how strictly this rule was applied across the centuries.