Perhaps handedness is a contributing factor. I imagine even amongst practiced typists, the non-dominant hand tends towards more technical/physical errors than the dominant hand. Errors might include descending early, late, drifting from the center of keys, a tendency to follow the sequence of the fingers, etc....

I come to this idea because I am left-handed and if I have made this mistake, it has happened infrequently enough that I can recall neither a particular event nor the fact of such events having occurred. I would be curious about the handedness of people who make this mistake frequently (enough) and those who believe they do not.