Can early ultrasounds be off by a few days?

My first ultrasound measured the sac at 5w2d and the fetal pole at 5w5d. Here's what happened:
​My progesterone level was 13.6 on aug 29th indicating that I ovulated, my hcg was neg. sept 4th my urine preg test was positive. My hcg levels from my previous pregnancy went back to 0 on July 21st. So sometime between July 21st and aug 29th I ovulated. I think I'm more like 5w3d now but could the dating scan be wrong?