About 3000 years ago, you only had wizards (who can cast any spell, but have to know it and prepare it), and a few clerics or druids (members of orders that advocated only certain types of spellcraft, and a greater emphasis on eithe religion or nature).
Then lots of bad stuff happened. Demons and celestials crossing the land, a huge war with an artifact that could mass teleport at will, and the first actual death of a dragon. All these things led to massive amounts of magical power filling the continent of Trema. Exposed individuals gained some minor powers, and their children and further descendants have inherited some of this power. Still, most spellcasters in the world are wizards, but a fair share of people have at least some touch of sorcery, but nothing they have the power or inclination to develop.
And there are still the options of being affected by fey or dragons and gained magical powers from them, and some of those who are controlled by the Trillith later develop telepathic powers, but whichever way you come across your power, sorcery always reflects the origin of that power.
Except in one case, but she's special.