Well, currently there are 20 players in NBA that average 9.0+ rpg, so, it's not like NBA is full of these players.
Only Utah (Jefferson & Millsap) and Lakers (Howard & Gasol) have two players in the list.
12 teams (Atlanta, Boston, Dallas, Indiana, Milwaukee, Minnesota, New Orleans, New York, Orlando, Philadelphia, Toronto, Washington) have zero players with at least 9.0 rpg.
Off those 12 teams, the team with the worst best rebounder is Toronto (Amir Johnson, 6.3 rpg). Washington comes second close (Emeka Okafor, 7.0 rpg) and Boston is third (Kevin Garnett, 7.7 rpg).
Yet, in the rebounding differential stats, Toronto is 25th and Washington is 27th in NBA, while Boston is dead last.
In the very same stat category, Indiana is 3rd and Minnesota is 5th, despite without having anyone over 9.0 rpg, too.
Numbers look confusing a bit, but all I can say about is that it's a team's job to be a better rebounding team. It's not like we're a team of midgets, but when you move KG to C, and expect Bass to have an impact on the boards, it's an unavoidable result that we lose the inside battle.