There’s a little list in my mind of information that dog trainers know and that they wish everyone knew. At the top of that list is the fact that dogs primarily communicate with visual signals whereas humans most often express themselves vocally. This difference explains so much of the confusion between our otherwise largely compatible species.
Dogs often pick up on visual cues that we use, inadvertently or not, when training them. So, if during training, we use a hand gesture while saying "sit," most dogs will learn that the hand gesture means to put their bottom on the ground long before they figure out that the word "Sit" means to do the same thing.
Research has shown that dogs learn visual signals faster than vocal signals. Therefore, it is most likely that if your dog is sitting when presented with both cues, he already knows the visual cue on its own. To check for sure, you can experiment by giving just the visual cue and see if your dog sits.
We often think our dogs are responding to what we are saying, but often they are actually responding to what we are doing. Dogs are watching us and we are talking to them. Dogs can't figure out what their humans are trying to convey and we can't figure out why our dogs aren't listening.
Simply being aware of this difference between dogs and people helps avoid the problems that often result. For more information, check out this short article I wrote for my local paper about visual versus vocal cues.