Welcome! My name is Melissa, and I am due Oct. 8 with twins as well. It is definitely overwhelming thinking about going from1 to 3 kids, but it will also be a wonderful adventure
I don't have any answers about measuring differently, but I wouldn't stress too much. My DD was always 6-ish days behind, and the twins are always 3-4 days behind as well. Early ultrasounds can also be pretty inaccurate, and once the babies start moving around, it can be harder! If your doc wasn't worried, I'd think everything is okay...one could simply have implanted after the other!