I'd assume the device version + iOS version + device ID is sent in the request and Apple maps this to the corresponding Siri version/database/API.. there would be no technical reason why iOS 5 can't see the same POI's as 6, only whatever business policy Apple has decided to implement. eg. freezing updates to the 5's database.
I'm sure the back-end processing is the same, but because 6 supports or will support new commands, then they are running separate instances & databases which results in varying update schedules.
As long as they don't restrict based on device model (only iOS version) then I'm happy with that.