It is well known the more herarchical a religion is the more underlying difference there is. Look at the roman Catholic church where local beliefs are mingled with ancestor worship, Voodoo etc. I am a former SDA and even though the church tries very hard for members to conformthere tremendous differences as some SDAs don't believe in the trinity, some churches have women clergy, some have practicing homosexuals as members and are active in the church.
The reason the witnesses seem to have unity is only because of there draconian policy of disfellowshiping and shunning. If these were taken away they would be like everyone else but with even more differences in there beliefs. The preoccupation on doctrine also feeds differences as people read the bible differently depending on background and prejudices.