By default, the recommendations are based simply on how long it's been since the last update compared to the average interval between updates. The original 15" MacBook Air was introduced well after the corresponding 13" model, so its average update interval is only 320 days compared to 386 days for the 13".
At the moment, that results in different recommendations with the 15" model being 69% of the way through its average update cycle while the 13" model is only 58% of the way to its average.
All of that said, we can manually override the automated recommendations, and since we expect them to be updated simultaneously going forward, I'll manually set them to both be mid-cycle for the time being. We'll likely shift them both to approaching end of cycle fairly shortly though.