It sounds like they're having some trouble pinning down a due date for you. I didn't have this problem so early, but with my second I measured at 36 cm when I was 29 weeks pregnant. Your fundus(uterus) should measure 1 cm for every week gestation you are, so I should have measured 29 cm. It was a bit disturbing, I thought OMG, how big is my baby going to be!?! At the next appointment it was only 1 cm bigger than it should have been. I think that everyone's body is different and as you know, they go by average.
It is possible that one baby can "hide" behind the other, so anything is possible when it comes to twins. I know how hard it is to wait, but I think you shouldn't worry about it and just wait to see what the u/s has to say. When you say quad test do you mean the alpha fetal protein? I don't know what the quad test is.