If you noticed, the page title says "Calendars", plural. In order to fully understand and appreciate how Eastern Orthodox fasts and feasts are calculated, and why Eastern and Western celebrations of Pascha/Easter differ, you need to understand a few basics.
Fixed Feasts depend on a particular date such as The Annunciation on March 25 or The Nativity of Christ on December 25. The Orthodox calendar year begins on September 1, and uses the same days and dates as the Gregorian Calendar used in Western Christianity.
Moveable Feasts are dependent on the celebration of Pascha (Easter) which changes from year to year. Briefly speaking, the date for Pascha is calculated as the first Sunday after the first full moon following the Vernal Equinox. The Vernal Equinox falls on March 20 most years, and occasionally on March 21.
Both Eastern and Western Christianity use the same calculation. However, they use different calendars! Western Christianity uses the Gregorian Calendar (circa 1582). All of Eastern Christianity continues to use the older Julian Calendar for the determination of the paschal date even though some Orthodox jurisdictions use the newer calendar for the celebration of their fixed feasts (e.g., the Greek Orthodox Archdiocese of America uses the "new" calendar for fixed feasts, but the "old" calendar for the determination of Pascha).
The difference between March 20/21 on the Julian Calendar and the Gregorian Calendar is around 13 days.
Besides the difference between the Julian and Gregorian calendars, the other difference in the determination of Easter between the Orthodox and other Christian Churches concerns the date of Passover. After the destruction of Jerusalem in 70 A.D. and the other tragic events which gave rise to the dispersal of the Jews, Passover sometimes preceded the vernal equinox.
As an alternative to calculating Easter by the Passover, "paschal (Easter) cycles" were devised. The Orthodox Church eventually adopted a 19—year cycle, the Western Church an 84-year cycle. The use of two different "paschal cycles" inevitably gave way to differences between the Eastern and Western Churches regarding the observance of Easter. Varying dates for the vernal equinox increased these differences. Consequently, it is the combination of these variables which accounts for the different date of Orthodox Easter, whenever it varies from the rest of Christendom.
In essence, Orthodox Easter (Pascha) will always occur on the first Sunday after the first full moon following the Vernal Equinox after Passover. This can lead to a divergence of celebration that may see both East and West celebrating the Lord's Resurrection on the same date, to as many as five weeks difference.
In order to assist faithful Orthodox Christians in planning certain sacramental celebrations, the Church will often chart important dates many years in advance. Since baptisms and weddings are prohibited on certain dates and during certain moveable fasts, this is a necessity.
Having said all of that, you may now be in a position to understand why the following chart has such different dates for the same event.
As mentioned previously, certain sacraments are not permitted on certain dates. The following are listed for your information.
Baptisms may not be performed from Christmas Day through the Feast of Theophany (December 25-January 6), during Holy Week, or on any of the Great Feastdays of the Lord.
Marriages are not performed on fast days or during fasting seasons or on the feasts of the Church as indicated: September 14 (Exaltation of the Holy Cross), December 13-25 (Nativity), January 5 and 6 (Theophany), Great Lent and Holy Week, Pascha (Easter), Pentecost, August 1-15 (Dormition Fast and Feast), and August 29 (Beheading of St. John the Baptist). Any exceptions are made only with the permission of the respective hierarch.
Memorial services may not be chanted from the Saturday of Lazarus through the Sunday of St. Thomas, on any Feastday of the Lord or any Feastday of the Theotokos.
If you have any questions regarding the selection of a particular date for a sacrament or memorial, please contact your parish priest.