Part of it is that our bigs our primarily jump shooters, Doc has employed offensive rebounding bigs before. Notably Leon Powe, Perkins, and Glen Davis (2007-2008 and 2009-2010 he got offensive boards well)
Being a jump-shooting big doesn't have to mean you're a bad offensive rebounder. See: Anderson, Ryan.
He's an exception, most jump shooting bigs don't cover the same sort of ground to get offensive boards.
Another factor is that the C's take a lot of long 2s, those shots produce fewer offensive boards compared to paint shots and 3s.