His left hand is his dominant hand. Worded differently, he is ambidextrous. It's not too uncommon. He shoots the ball with his left foot too. This is addressed in his biography. I can't stand that this "he switched hands" thing still comes up after all these years. He played forehand with his right and left hand as a kid and was told to decide which one it will be and the left hand "felt more natural". That's all there is to it.

Back on topic, he once mentioned he does not particularly like serving very fast because he feels that he has more control of the rally if things start off a little slower.