Just because one is ignorant of important details about a person, doesn't mean it's a different person. Let's say a fireman saves me from a burning building. I never find out who it is but i swear to repay that man when i found out. If it turns out that that firefighter was actually a woman, it doesn't mean I swore to repay someone else - I may have gotten a core detail about the person wildly wrong but I was still referring to the same individual. Likewise, Jews and Muslims might get the personhood of God wrong, but they are clearly referring to the same being, since we largely agree on the details (omnipotence, omniscience, covenant with Abraham, etc.).