I've only just started flying the Airbus (319/320/321) and so what follows is based on minimal experience on type; however, it could be the small difference in eye-height that may be to blame. In the 321, your eyes will be a little higher above the runway on touchdown than in the 320 (due to the greater distance between the cockpit and main wheels). Another factor may be the flap setting. Are you using the same for both types consistently?