After seeing some people talk about this in discord, I set about testing how this works. While the Galactopedia says that high levels of suitability should increase max population above the estimated Max Pop size, what ISN'T addressed is that even with a suitability of 20, the achieved population can be as low as ~42.86% percent of what the max pop size should be.
Suitability of 20, non-native planet type (but still preferred with 10% bonus)
But the theoretical max pop of this planet is a full 2827m. If you divide the actual max by the theoretical max, you've achieved only 60.03% of the quoted max.
However, this actual max being far below theoretical only occurs for all planets that are not your homeworld's type.
I edited my homeworld to have the exact same +20 suitability as the Sandy Desert Planet, and as you can see - the achievable max is higher than the theoretical max, which is consistent with the Galactopedia entry.
The same thing occurs with native planet types that are not your homeworld (Rocky Desert type)
Population of 4500M above the quoted max of 3927M for native planet type
But take an extreme case, where you colonise a Grassland planet of Size 6000 and quality 70% (giving 20 suitability for Mortalens). The achieved pop max is only 42.86% of the quoted maximum.
I'm presenting this as a bug - because this functionality is not documented or covered anywhere and I fail to see how this would be WAD. Save game is attached, check home system and look at the three planets.