Sure, I'll give it a shot.

When you post to either chainBB or Steemit, they both submit the Steem blockchain, and are shared on both sites. The steem blockchain is what's generating the post rewards, independent of whatever site they are posted on.

Both chainBB and Steemit have different mechanisms for "surfacing" content (i.e. displaying it to readers). Steemit uses a model that is dependent on upvotes and weighted by the voters, then surfaces the content much like reddit would, where the most weighted/upvoted content gets displayed most prominently. chainBB on the other hand doesn't care how many votes a post gets, and the posts that are displayed at the top of each forum are the posts that have been active the most recently.

With those different mechanisms in place - they both help different kinds of content earn rewards in different ways. Steemit creates a "dog pile" effect, where once it's trending, everyone else jumps on, and helps those big posts earn bigger rewards. chainBB on the other hand keeps surfacing posts that are actually getting comments, which helps the posts with active participation earn rewards.

So while both of these sites are based upon the same set of data, one is focused on surfacing the best content (Steemit) and the other is focusing on surfacing the most active content, by category (chainBB).