need some input on due dates

Laurel
Probably a stupid question but every time you go to an ultrasound and your due date changes will it ever be like sooner? 
By my LMP I was June 13th. But I know when I conceived. So going again I measured due date to be June 21st at 6w6d and now going back I was supposed to be 9w6d and the baby measured 10w1d. So my due date is now June 19th. I'm just curious as to why it keeps changing which one would be right. Etc.