Have you ever been in combat and heard men crying out for their mothers? They know she can't intervene, but they do it anyway, because it brings them comfort, and it is a natural thing to do. I believe it is the same type of thing with Deists praying.
Washington read his Bible and prayed an hour each morning and an hour each evening, even when not in battle. I don't know of any Christian today who is as faithful as that. By the way, have you read any of Washington's prayers? I have. They often speak clearly of the Trinity of the Christian god. That also is not what a deist would believe.
Finally, I am not saying that Washington was not at all influenced by deism. He probably was; just as Christians today are often unduly influenced by the philosophies of our times.