I knew exactly when I conceived both girls so I knew what my EDD should be - the early scans were out from my date by 1 day each time (so neither here nor there). It never changed during either pregnancy. Having said that neither baby arrived on the EDD.

I think if you have no idea when you have ovulated and have taken a while to find out that you are UTD (eg. you are 12 or so weeks along before you realise) then I can understand why the dates might change but it is a bit strange to just keep moving dates based on the size at an u/s.