Measurements, pros only, please!

Allison
this is mostly a question for my nurses and ultrasound techs out there. So I had an u/s on Friday, when I should be about 9 weeks along from LMP. Crownto rump length is right on target at 8w6d. However, the gestational sac is only measuring 7w2d. Why the discrepancy? Is it just stupid pregnancy math that doesn't make sense, or should I bring this up to my ob?