I think it is all about the variation in our bodies. According to my LMP and even my OPKs, I know pretty exactly when I ovulated. I shouldn't have been able to test until the first week of December, but tested November 30th and got a positive on a digital. The fact that I tested a week early and got a positive, plus how my symptoms are going I feel like I am about a week a head of where all the books, websites etc say I should be but I don't get my first scan until February 1!! Its driving me nuts. So either my body responded with hormone surge REALLY fast the egg implanted the second it got to viable uterine tissue. Don't stress about the how many days etc. As long as baby is growing and has a healthy heartbeat that is what matters. Remember the measurements are averages. Yes most babies will fall into the averages, but some babies grow fast and some take their time. Some families have short genes, others tall!
My daughter was considered full term, but she was only 5 pounds 8 ounces, but even that was almost too big for me to birth vaginally as I am only 5 ft tall. So please remember to trust your body and your baby. Sometimes I think this age almost gives us too much information!