To increase the Stewards of Hades achievement Level, your Coalition needs to capture and hold X mi.² territory for 15 minutes.
The game rounds up your Coalition's controlled territory to the nearest number when converting from thousands to "K." Even though your Coalition is currently controlling, for example, 2975 mi.², the game will display it as 3K mi.² on the Map and in the My Coalition window.
However, in the Stewards of Hades information, the game will display your Coalition's territory as 2.9K mi.², not 3K mi.².
You're almost there! Just a little more, and you'll get to the next achievement Level!