Instinct 2x vs Fenix 7x

The Instinct has 40 days in smartwatch mode vs 28 days for the 7X. (without solar)

How is it possible that the Fenix has 63 hours gps all systems and the Instinct only 40 hours? 

I would think the instinct 2X has also better gps autonomy.

How is this possible? A cheaper gps chip who uses more battery?