Anyone ever have their ob get their due date wrong by like 2-4 weeks?

My last cycle was 12-22-16 I took my first test 1-22-17 and got a very dark positive and then a digital test said pregnant within 30 seconds of peeing on it and another digital test said the baby was 3+ weeks. I had my first ultrasound the next week I was around 6 weeks we saw sac and baby just no hb yet and I was estimated at 5w5d and told to come back in 2 weeks for a definite due date. At my 8 week appointment the ultrasound tech had a lot of trouble getting the wand to where we could see baby (never got a good view) but heard the hb a little and is measured at 147 bpm. She told me I was only 6w3d. How is it possible to see a sac and baby when baby is only 2 weeks along and get a very dark positive almost a week and a half before that?!