Why does Orthodox Christmas "lags behind" Catholic one by 13 days?

During studying at VCC, I have been questioned many times why people from my culture do celebrate Christmas in January. I am a Ukrainian, and my culture belongs to the Orthodox religion.
This difference occurred because of confusion with calendar in 1582 when was introduced a new Gregorian Calendar by Papa Gregory XIII. That calendar was dubbed as the New Style.  The old, Julian Calendar, was called the Old Style. The discrepancy between the New and the Old Styles of every one hundred years increases by 1 day. Therefore, there was the difference of 13 days in the beginning of the XX century.
At that time in Europe was used the new Gregorian calendar, and in Russia people continued to use the Julian one. The government of Soviet Union introduced the Gregorian calendar in 1918 but the Church did not approve that decision.
At the initiative of the Patriarch of Constantinople in 1923 was organized meeting of the Orthodox Churches, which adopted the decision to correct the Julian calendar, and there was created the New Julian Calendar. Due to the civil war Russia couldn't participate in the meeting. However, the Patriarch Tikhon issued a ruling on the transition to the New Julian Calendar, but that step led to protests among religious people, and the ruling was overturned.
Since then, Catholics and Protestants who live according to the  Gregorian calendar have celebrated Christmas on the night of December 24 to 25; the Ukrainian, Georgian, Russian, Jerusalem and Serbian Orthodox Churches have celebrated Christmas on the night of January 6 to 7. They are living according to the Old Julian calendar as well as many Eastern Rite Catholics, particularly, the Ukrainian Greek Catholic Church and the Russian part of Protestants.

There is more information about calendars and the astronomical year, but it's very complicated.
- The discrepancy between the Gregorian and New Julian calendar  in 1 day will have accumulated by 2800.
- The discrepancy between the Julian calendar and the astronomical year in 1 day is accumulated for 128 years.
- The discrepancy between the New Julian calendar and the astronomical year in 1 day is accumulated for 40000 years.
- The discrepancy between the Gregorian and the astronomical year in 1 day is accumulated for 333 years.


Comments are always welcome :-)