One of the developers has been under taking a few changes to how modules work. By default now it doesn't show unsupported modules (modules which are not currently maintained by a developer) so that 3 pages your seeing is the official SilverStripe and community supported modules. The rest you'll have to tick the unsupported box. I believe this 'Supported / Unsupported' option is selectable by the module developer.
Could you explain this a bit further. What qualifies a module as 'maintained by the community'? How does SilverStripe keep track of which are actively maintained? Would it be too much trouble for SilverStripe Ltd to contact maintainers enquiring as to their status before hiding their projects?
I agree with the sentiment of highlighting quality actively maintained modules, but this seems a bit arbitrary (or, at least, at the mercy of persons unknown at SS Ltd).
Edit: Ahh - ok the "what does this mean" link was either hidden or too tiny on Mobile Opera :D.
Hmm, there appear to be some issues with the list. Secure Files appears as "Community" but only shows up in the "not supported" search. Also, my Spam module is supported! At a guess, is the the 'compatibilty' fields that prevent these being marked as supported?