Dating scan - gestational sac or CRL??

I’m currently somewhere around 7 - 8 weeks. I had some bleeding and an ovarian cyst has been discovered so I’ve had a couple of scans to follow up on all that.

Anyway I’ve realised that the OB who is undertaking these scans is using the size of my gestational sac to date the pregnancy (putting me 1.5 weeks behind where I thought I was) rather than the CRL which would put me almost spot on where I thought I should be.

I’ll be seeing my regular OB in a week’s time so will ask him then for confirmation but has anyone else experienced this? Or know why they would be relying on a gestational sac measurement to date a pregnancy around 7 - 8 weeks? The embryo is there and they got good CRL measurements so I’m a little confused. I’m also just a little bummed as I’ll be having the NIPT / Harmony blood test at 10 weeks so putting me back a week means I need to wait another week 🙈 I’m super impatient if you can’t tell haha.

Thanks!