Actually most of the responses on here so far have been wrong. :) Both dates are probably not right "to the day". At 6 weeks an u/s cannot detect to the day with certainty. I believe it's give or take a week in either direction, actually. If you were to have moved, if the baby had moved, if the tech had moved the wand during the measurements the date would be different.
However, LMP is not always right, either. Some women ovulate earlier and others later than CD14 which is what the LMP is going off of. Some ovulate much later, like CD30 or more which makes LMP over two weeks off. The best way to tell how far along you are is knowing when you ovulated, but if you don't have that info the second best way is the NT or Anatomy scans which are more accurate since they typically take measurements of several things over a longer period of time to get a better idea of what's going on in there, basically.
At the stage you're at, to figure out when you can expect the baby take the latest due date you've been given and add two weeks to it, and you can expect the baby to come sometime in that window. Most places won't let you go over 42 weeks so you likely won't go past that.