Help! Sac vs. Baby

I had my sonogram today and the baby measures to be 7w5d but the sac measures 8w3d. They said that's normal and heart rate was 157bpm which they said was great but Ive never heard of this difference in age between sac and baby. Does anyone have any info or personal experiences with this? They told me it's absolutely normal but should I really be freaking out? Also because of this my due date is uncertain. They said to go based on baby's age so due date is December 10 but then that would make me 8 weeks as of the day. So confused! Thanks in advance!