How is it possible this is my due date?

My last period started January 1st. I had on the 14th and on the 21st and that was the last time until March. I found out I was pregnant in Feb. The 12 week sonogram predicted my due date was October 18th instead of the 8th, like I calculated, based on the growth of the baby up to that point. THEN at my 24 week sonogram, the baby measured (in 2 seperate sonograms) like a 28 week fetus which placed him in the 95th percentile. I'm thinking, is it possible at the beginning of the pregnancy, that the fetal growth was slow?? and what is my TRUE due date??