Usability Beef Bourguignon

Dear Microsoft Word usability team,

When I select Format | Styles and Formatting in Word, the following pane appears on the right of my document:

So I'm looking at available formatting, which is only 3 styles. So what is the dropdown control for? Let's see...

Aha, so there are "Available styles" and "All styles". So what's the difference?

Shudder me timbers! Apparently, there's a whole bunch of other styles, but they're somehow not "available". Whatever that means, 'cause they sure as hell are available now.

Can you guys explain to me what the hell this is about?

Kind regards,
cronopio.