Good point but still a buy. I need to check but I

Good point but still a buy. I need to check but I had a limit buy order set at the ask. My bet is the broker gave a fill at the bid which is cheaper for me but where did the shares come from? That was definitely not a retail sell order I was buying. It filled almost immediately on very low volume. I expected the order to be sitting for a while i.e. waiting for a sell order.