I saw many pornstar place the ruler under the cock (where they can go much behind the pubic bone) or "hiding" the first digit under their thumb, so they can start from 1" instead of 0, some other are never publicly measured.
Many of them are also paired with smaller guys and we also have to remember that a lean framed guy or a short guy (5'5") with a 6,5" cock could seem hung and a 6'3" with a big cock just normal… even in that case we don't know their real measure.
And then there are overweight people (like me, but I'm trying to lose my fat) that have obviuosly fat even in the pelvis... i have at leat 1" on cock hidden by that at the moment... some other have a curved dick (yup... me again...) so it's hard to know how to measure it (tryng to correct the curve or following it), and some other have an oval cock instead of round, so if you see them from the front it seems a beer can, but from the side a pencil... some are showers and hang down at the same size they have erect other are growers (me) and when in underwear seems non-existant (so if you haven't had sex with someone but only saw in lockerrooms you can't say if they are bigger or not)...
There are indeed hung pornstar, but many of them are just well framed in the camera (perspective can do miracles)...
Unfortunately our eye is very bad at comparing and if your mind is convinced that you are small your eyes will tend to understimate.
The only way is standing right to your favourite porn star and realise if you are better or worse, but I don't think is something so easy...