why is our cross different ...what do the extra features represent
To answer you second question: The first bar represents the sign that was placed above Christ with the inscription "King of the Jews" in ( I think ) Aramaic, Latin and Hebrew letters. The bottom bar represents the "foot" rest that was tilted when as tradition has it, the spear pierced Christ's side.
As far as why the Orthodox adopted the 'three bar cross' I will leave that to others. One think I can tell you is that it is exclusive to the Eastern parts of Europe especially Russians, Slavs, and Ukrainians.