[dore]

There are a couple reasons why this could happen:

1) You play by ear more than you play by sight for timing purposes. So you prepare yourself for the note but wait for the sound that indicates you're supposed to hit that note. You wait for the sound to start the button press rather than anticipating and having your finger down when the sound plays.

2) You play more by sight than by ear. You see a note and wait for it to cross the target to hit it, so you start your motion when the arrow crosses the target rather than in advance.

The moral of the story is anticipate more; lay back less.