There are three men wearing a hat here: "Ruby", Lavealle, and the guy on the right. Notice on Leavelle and the guy to the right of that enormous guy in the center, that their hats are well above their ears. But, on "Ruby", the hat is pulled down all the way to his ears.
He's wearing it very low, too low, and he did that to hide his face as much as possible. Now, Jack Ruby could not have done that. That's because Jack Ruby had no plan at all. If you listen to him on tape, he went there with no intention of shooting Oswald. It was a last minute, spur-of-the-moment impulse, according to him, which he did as though he was in a trance. So, what are the chances that he would think of pushing his hat down like that? No, that was done because that guy wasn't Ruby, but just pretending to be Ruby, and the low-riding hat was effectively his disguise.