When I click either (they both do the same thing for me) What's New or New Posts I get a linear listing of all forum threads that have been recently posted in. If I click one of those threads to read it, and then click What's New or New Posts again, that thread is no longer there.

In other words, I only see threads that I have not read.

Other forums show you all active threads. Usually, the ones you have read are normal font and ones that have new posts since the last time you read it are bold font.