6 week ultrasound

Currently I’m 6w5d and I had an ultrasound two days ago when I was 6w3d. The nurse practitioner measured me at 5w5d, we could just barely make out the heart beat (and heard it) but she’s having me come back next week for another ultrasound. I didn’t ask why because she didn’t sound worried so I decided not to worry but I’m just curious if anyone has had an ultrasound this early and had to go back in? Just curious if it purely because it was so early and she wants better measurements or if measuring 5 days less than what I am is an issue?

I didn’t even know I was pregnant with my son until I was 7 weeks and there was a clear heartbeat and baby.

Just curious!