It also probably stems from the fact that so many religions have evolved so greatly over time. For instance, and as I believe you mentioned, modern Christianity often bears little resemblance to early Christianity...and this is without considering the dozens, if not hundreds, of sub-denominations that have sprouted up in Christianity. There are many times that I will look at one Christian Church compare it to another Christian Church, and almost wonder how they manage to both call themselves Christianity.
I guess it might also come somewhat from my integration of various theological 'bits' from other groups into my own 'dialect' of (sorta) Christianity. If someone suggests something that makes sense to me, I'll not ignore it simply because nowhere in the Bible or [insert other religious document] is it stated in the exact same way to be truth. If it clicks, I'll take it, and I'll incorporate it into my beliefs.
