Exactly: muscle memory. In your car, do you move the turn signal lever up or down to indicate a right turn? It actually takes longer to answer this question than to do the action in real life.
Another example: There are several passwords that I can type instantly, but I wouldn't be able to recite them.
h is left because it is on the left of the movement keys, l is right because it is on the right of the movement keys... a fairly convenient correspondence.
Just have to imagine the motion. I remember left/right by literally right clicking & feeling my middle finger be to the right of my index. I got a little mixed up when I switched my mouse to my left hand
Another example: There are several passwords that I can type instantly, but I wouldn't be able to recite them.