Some Orthodox churches require from the faithful a tithe while other require a pledge. Why is that?
While our belief in Christ and the Apostolic Witness is constant, there are many small differences in the various Orthodox jurisdictions, especially in the area of administrative matters like collections. Some churches pass baskets around during the Liturgy. Others have the basket at the end as people leave. Different parishes often have different practices.
In America, things are different than they are in the "old" Orthodox countries. In many Orthodox countries like Greece, the clergy and support to the church is actually paid by the government through revenue or taxes, so people do not develop a strong sense of supporting the Church financially. It is done indirectly. Here, we have many converts from Protestantism who have a much stronger sense of responsibility to the Church and no government support. So Protestant terms of "stewardship" have become popular in many parishes. They emphasize that we ARE responsible to regularly support the Church, not just at Christmas and Easter, but through the entire year. Therefore they institute the concept of a pledge, which simply means a commitment from the parishioners to provide a realistic level of regular support, while people from the "old school" only give a small amount, which is not enough to meet the financial obligations of the parish. Buildings need to be heated and maintained, priests need to be paid, and so on.
Some other parishes, also Protestant influenced, instead of calling it a pledge, use the Biblical term of "tithe" which is a Hebrew practice, and is somewhat controversial in some Orthodox circles. "Pledge" parishes generally operate from an idea that the old Law no longer applies. They would say that God does not want just 10%, He wants 100% of our time, talent, and resources. We give whatever we can in the circumstance we find ourselves in, be it 1% or 80%. Tithe is a "fixed" amount regardless and implies that if we give that much, we have met our "obligation" and we don't need to give anymore. Others would say that tithe is more flexible than that, you give whatever you can. So in the end, they both become simply different words for the same thing: as Christians we have an obligation to support the Church and charity as best we can.