It's considered 'available' because Standby pages don't have to be written to disk before they can be assigned to some other use. page faults to these can be resolved without going to disk) and also for proactive file cacheing by SuperFetch. It's being used as a page cache (it holds pages recently lost from all process working sets i.e.
If it helps, Resource Monitor describes all the other RAM as 'Standby'
