Okay, that makes sense what you're saying, about them looking the same from the outside. But we don't really know the conditions about "initial belief." Islam started 600AD, which is too long after Abraham's faith to declare dogmatically that the initial belief is the same.
Also, I'm not talking about merely differing interpretations. I'm talking about a basically different view of God (the Trinity), and a different way to be reconciled to him. There are core differences between the religions.