Did you have a scan at this time with your other two children? It might be that they were exactly the same and that its just whats normal for you.
My first DS was over 10lb and his brith itself was pretty uncomplicated. I had a scan at 36w to check the size of a fibroid, and the sonographer said 'Um, whens your due date' I said in a month and he says. 'Oh..are you expecting a large baby?'
I said id be surprised given my birthweight and Dhs and family history, if my baby was under 9lb. HE says oh thats good then cos this scan says your due today based on size and measurements!
Way to stress someone out!
When I was pg with DS2 my midwife said unless there was a serious medical need, I should avoid any late u/s becuase they would only result in the same thing and drs pusing me to get induced early, when we new Id birthed a 10lb bub with no dramas.

Anyway what Im trying to say its its all probably just normal for you. You are right, they are only estimates (mine were pretty accurate though).

I know my OB told me he is very hesitant to induce early or recommend late scans for growth becuase he'd had 2 where they suggest > 10lb bub for two rather small women, both were induced early, one unded up with a c/s and both bubs were < 8lb.