Great news! a heartbeat is a heartbeat at the end of the day. My dates didnt add up at my scan either, I was moved back a week and a day despite my cycles being very regular. I havent been moved forward at any of my subsquent scans either and I've had a few. Try not to worry, since you've had a heartbeat confirmed you should try and relax and enjoy your pregancy (easier said than done I know)
xx