All of the BangBros "fuck a fan" things are fake. They find amateur male talent off Craigslist, which is why some of the guys can't keep it up or don't look so great.
You're mixing two different porn productions together. "FuckTeam5" is Bang Bros. "Fuck a Fan" is Immoral Productions. Both have a similar theme.
I can tell you for a fact how it works: "Fuck a Fan" has used true amateur performers but mostly professional male talent with "low porn miles" (not easily recognized or famous). However, most of the guys are "real" porn actors. There's a couple of true amateurs who sneak in from time-to-time, but they are usually the ones who can't perform, and honestly, it makes for a lousy production and wasted investment (the shoots aren't cheap).
"Fuck Team 5" tends to use more "true" amateurs, and that is why so many of these scenes result in two inch cocks that never get hard, and a gaggle of laughing porn girls. However, the scenes usually have multiple guys. So, they throw in a "real" porn guy here and there to "show-up" the true amateurs. "Fuck Team 5" almost taps into the humiliation niche whereas "Fuck a Fan" isn't focused on that, and is more one-on-one sex with porn girls who are supposed to be enjoying themselves, as opposed to humiliating the guy. "Fuck a Fan" is more the fantasy of an Average Joe who turns out to be a competent porn stud if only given the chance... and the porn girl, surprisingly, enjoying the unexpected surprise.