I’d say they are following usual pattern. The Air is going down slowly and that’s normal as it is using way less energy then the Sky …
The sky typically dives at some point as you can see higher in this thread.
Maybe winter period adds to a more acute drop as every battery is sensible to temperature.
I’d say, start buying a new set for the sky as it will last a bit more but will drop quickly once they go under 3V. Some hang on a few weeks or like mine, 1 week later and game over 
here is a graph of typical behaviour of the Energizer