When you terraform a Sterile planet, the tooltip displays the wrong Maximum Population if you have already constructed Eco Habitats (+1 Population slot on Sterile) in the system.

See screenshot below, tooltip shows a Max Pop of 7 (+2), whereas a Small Savannah should have a Max Pop of 6. No other modifiers are active on the planet. The tooltip doesn’t take into account that Savannah is not Sterile, so the Eco Habitats buff won’t apply anymore. (Without Eco Habitats Savannah indeed has 2 Max Pop more than Desert.)

Verified this is just a display bug with the planet Dobrynya I to the right, which I terraformed from a 4-pop Desert to a 5-pop Savannah.