the amount of volume and/or intensity they can recover from?
Secretly, no one wants to admit they're a pussy in the gym. Mostly they go one of two ways-
a) They think "Kazmaier, Arnold, the Bulgarians, etc.......... were beasts in the gym, if they can handle it, so can I" .
b) They latch on to Mentzer style HIT or Hardgainer philosophies- "I'm overtrained and just not good enough".
On the body fat thing, some folks are just liars, others simply don't have an honest idea what a real 5-8% bodyfat looks like. You really have to look at a few honest photos and tests before you get a feel for it.