you could have ovulated earlier than thought ... but hey when was you last period? Do your dates add up? as some time these u/s can be inaccurate as well.
I know as my pregnancy dates were a month out between when i though my baby was due and when the u/s thought he was due. Mine was July 12 edd from LMP and U/S was July 31 and i had him June 29 he was perfect! So go with your dates! xx