Lmp says edd 7/24/14 but first vag u/s says 7/20/14 does that mean i ovulated 10days after lmp rather than 14days?

U/S not perfect: The error in an early first trimester u/s is +/- 5 days. If the u/s measurements calculate a due date of 7/20/2014, it means your "real" due date could be as early as 7/15/2014 or as late as 7/25/2015. Your LMP due date is within the accuracy range of the u/s. You are correct that LMP is somewhat imprecise. Not all women ovulate 14 days after the first day of your period.

No: The edd given with vag u/s is an average of several crown-rump measurements. It gives an edd with a range of 3-5 days usually. If a woman has cyclic menses and the vag u/s is within 3-5 days of the edd by lmp, then the LMP date is used. Vag u/s dating is best used when there are discrepancies in lmp.