Is ultrasound dating always right??

Kayla • Due Oct 13th with baby girl #2!!

Got my bfp on 1/26 at what I thought was 11 dpo. Pretty sure I ovulated on the 15th. From my calculations I’m 7w. Went for my ultrasound today and she says I’m 6w2days. Thought my due date was 10/8 but now it’s 10/13. If I ovulated 5 days later (20th) then how the shit did I get a positive on the 26th?? I’d have only been 6dpo!