Hey Ollie,
I wouldn't try to read too much into it. As far as I know the 5oth percentile is good, means bubs is smack bang right on the average size for his age. But like Michelle said, scans can also be a little out, so he could be bigger than the results say. But just remember that the percentiles are only done on an average range. It takes smaller babies and bigger babies to create these average measurements, if that makes sense. So what I'm trying to say is, that all bubies are different and I'm sure your bubs is as happy and healthy as the next.
BTW I'm jealous I wish I could get to see bubs again.