I'm not positive it is.
If you are in Canada and ask Siri for something like "where's the nearest coffee shop", in iOS 5 you get "I don't support that in Canada", but in iOS 6 you get the expected results.
If you are in Canada and ask Siri for something like "where's the nearest coffee shop", in iOS 5 you get "I don't support that in Canada", but in iOS 6 you get the expected results.
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.