It depends very much on how detailed they do the US.

Some just do a measurement of the Femur and a couple of other minor measurements and these estiamtes can be out by quite a bit.

We have had some done where they take a lot of mesurements around the abdomen, head, limb lengths and diameters etc, to this detail the weight is a lot more accurate, but never perfect

I wouldn't worry too much yet because a lot of people have been told they are having a big bub and a little bubba comes out!
Take care and glad that everything was ok after your bleed