5 offensive rebounds is pretty good as long as they're not the Al Harrington variety. (actually give credit to Al for not giving up on the play) Maybe they were long rebounds off of missed 3's? Usually guards get those kinds since they play away from the paint.
DeMarcus Nelson is pretty athletic (actually, very athletic) and he's got good size too. On top of that, he's very active on both ends of the court, so I wouldn't put it past him to say he earned those boards.
I guess being non-guaranteed gives one incentive to just go out and hustle it down over those spoiled full contract players.