The question raised is a good one. I had the same question with my own experience. The disagreements were there even in Buddha’s time. This is why I think that we have disagreements.
a) Not understanding or misunderstanding of Buddha’s word.
b) Trying to interpreted Teaching literally rather than through experience
c) Do not want to accept Buddha’s word as final. For example Great Vegetarian debate.
d) Doubt – one of the Fetters
e) Mana – one of the fetters least understood by many
f) Not serious about Dhamma but just to have a conversation
g) In Buddhism no external agency to put the blame on things. We are responsible for our own action. This is very hard to accept. We always think that it is someone else fault
By attaining Sotapanna all this doubt will come to an end!
“As the lamp consumes oil, the path realises Nibbana”