It's because that's your baby's gestational age. I am currently 39 weeks plus but my baby's gestational age is only 37. Most people go by when the last menstrual cycle was, so off of that, you are 10 weeks pregnant. Hope that makes sense to you.
That happen to me to. But I was 26 weeks
Why are you disappointed? You're still pregnant, just a few weeks behind than what you thought. Be happy baby is there!(: