I am wondering exactly the same. Maybe the standby costs? The chip needs to always have power to be able to receive the radio waves, it uses less power in that way but still quite a bit, I guess.

The surprise is how much power that is. It's either that or misleading marketing. I know Apple struggled to make this modem, so maybe it's still not great when it comes to standby power consumption. Since it's Apple we will only know after quite a bit of independent testing...