6w2d measuring at 5w

Mackenzie
So because of two mc this summer, my midwife wanted to get an US done when they thought I was around 6w. Apparently I'm measuring at around 5w. 
I had ovulated exactly 4w2d ago (used opk) and my lmp (which was actually my last mc) was exactly 6w2d ago. Crazy how the dates can be so different. Anybody know why/can explain? 
Anyway, I'm happy that things look okay for 5 weeks, even tho I was hoping for a baby and heartbeat today! Going to get another US in two weeks.