Ovualtion off?? A bit confused.

Going by my LMP I should be 7+4... I was 3 days late and got my 1st Bfp. Today at the Dr during u/s we discovered it looks more like I am 5 weeks. Which kinda makes sense bc we have thought I've had low hcg numbers..until now. My question is how in the world did I get my Bfp is I ovulated only days before. I'm a bit lost. I had more labs down today find out numbers tomorrow. They were 3000+ about a week ago.