I've a lvl 1 in reb and he has 2,5 reb per match as average, a lvl 1 in passage has around zero assist per match
Like Kukoc said, there must be rebounds assigned in every game. Assists are created by pure passing skill, rebounds are created by missed shots. If you have no passing ability, you wont create any assists. If you have no rebounding skill, there are ~50 missed shots a game and one may bounce your way.
In addition, there is a HUGE difference between a low level 1 and a high level 1. I have a player with low level 1 passing. He has 1 career assist in ~10 seasons.
One final point: I place a large emphasis on rebounding for my big men. I usually win the rebounding battle overall. Sometimes a lower level rebounder grabs 20 boards and my best only grabs half that amount. It may be the equivalent of why your respectable-inept shooting center went 5-5 behind the 3 and your Prolific-Proficient SG went 0-5...matchups.