You did read my quote, right? Let's shift the bold around a bit.
90 is . . . the maximum basic population . . . a complete collection adds 5 to this. Completing all collections adds another five, for a total population cap of 115 . . . Magic Faction, level 3, boosts population by ten more, for a theoretical population cap of 125.
If both tribes have the third level of the faction, and they're at equal max population, obviously the collections are the difference. Let me guess; you have two collections complete on your Nature tribe, and none in the Magic tribe?