Confused about conception

Helen • I am 30 yrs old. Been with my partner 5yrs. He is a guitar teacher and I am a home carer. Bought our first house last year. Now have our first child. A son named Harvey born 27/10/3016
I have just had my first scan and I was told I was 11 weeks and not 10 weeks 5 days, I thought I conceived on Feb 15th as my ovulation tests got a positive on the 14th but this doesn't match up with me being 11 weeks so how could I conceive 5 days before I got my positive ovulation? I am so confussed!! We used the every other day method so I can't really narrow it down that way.