A. The difference in calculating the date of Easter is due to the use of two different solar calendars; i.e. Julian and Gregorian. In 1582, Pope Gregory XIII adopted the Gregorian calendar for all Catholic countries to correct a weakness in the Julian calendar which proved to be 11 minutes too long which over centuries turned into days thus altering the important date of the spring equinox. In doing so, Pope Gregory the XIII also removed 13 days from the calendar that year to return March 21 as the date of the equinox.
Traditionally, the date of Easter is determined by the first Sunday after the first full moon after the spring equinox. This was to keep it as close as possible to the Jewish celebration of Passover on which Jesus was crucified and risen. However, since March 21 is no longer the same day according to both calendars the calculated date of Easter is usually different for Catholics and Orthodox. There are some occasions when the dates do line up on the same weekend like 2010, 2011, 2014, and 2017.
While the discussion of having a common date for Easter has been discussed and promoted this has not yet taken place. Discussions and dialogues are ongoing.