One of the things that surprised me a number of years back in regards to porn stars was that many of them are short guys. Being short, their cocks look proportionately larger on them than the same size cock on a taller guy. I met Jeff Stryker when he came to the club I was DJ-ing at, and he barely came up to my shoulder (I'm 6'2"). With that in mind, I wouldn't be surprised if he actually measured out more than 8", though on him it looks frickin' huge.
It is for this reason, and because the nature of the porn business is all about creating fantasy, that models are often assigned sizes that are in excess of their actual penis size. The little guy with an 8" rod looks like he has 10" on film, so the studio says he's hung with 10".
Sometimes I would just shake my head while leafing through the pages of INCHES magazine when I'd see these figures of 9 and 10 inches assigned to guys whose cocks barely reached their navels. But as I said, that's the biz. If they can make you believe it and it helps sell magazines and videos, all the better for them.