I agree with what's been said so far

When I was pregnant with Tristan they added 12 days to my LMP due date.... at every single u/s he had caught up more to what would have been my gestation based on LMP, he was born at 34+3 weeks and 3.09kg, the average 36-37 weeker.

U/S are not 100% spot on and can be out. Also as someone else pointed out not all babies grow at the same rate.

It's great they will be monitoring you a bit more, hopefully just as a precaution and nothing else.