DH and I didn't DTD much after I O'd so I am quite sure I couldn't have fallen pg from those times.
Only takes once though, perhaps in one of those few times you actually did fall preg.....
As people have said u/s is only as good as the sonographer. Dates can easily be out by a week either way. I knew the exact time of conception for this preg as we only DTD once! My scan was a week different to the date of conception. So there is clearly a little variation. But i wouldn't expect 2.5 weeks difference. Maybe it is just somewhere in the middle.

Nice to have a few extra weeks up your sleeve though.