Strong heartbeat but slow growth?

Based on my LMP, I should be 7 weeks 4 days. I’m almost positive we conceived on 8/14 (we only had sex the 11th and 14th), so “technically” I should be around 5 weeks.

I was seeing a specialist so I got an ultrasound at 5 weeks 4 days, couldn’t see much but was measuring appropriately and there was a yolk sac and everything.

I went in today, 2 weeks later, and was only measuring 6 weeks 1 day when I should be measuring 7 weeks 4 days. However, the heartbeat was an amazing 163.

I’m so confused... can there be a strong heartbeat with slow growth?!