You’re not misunderstanding it, it’s basically an unecessary extra step where they convert your stat into a point then into a percentage.
They could just have easily directly converted the mastery stat into a % based on each spec.
I think the main reason they did it was because a single mastery stat would convert into a miniscule mastery %, in SL for example (because I don’t know the conversion rate now), you needed 35 mastery stat to make 1 mastery point.
So back in SL, you’d end up with 1 mastery stat giving 0.0214 Ignite%
I find it even more hilarious that fire mage has to take talents to boost mastery, whilst frost spec with exact same gear and zero mastery talents I get more mastery.
Fire and Frost have an equal number of talents that buff mastery.
Frost’s talents focus on directly or indirectly buffing Icicles without giving extra mastery stat, while Fire has 3 that give temporary mastery % and two that passively (and invisibly) buff Ignite’s damage.
There’s not a huge difference, I’ve got 19% as Fire and 26% as Frost and that’s without From the Ashes. I can temporarily buff my Fire mastery to 41%, while Frost’s mastery is fixed.
The main thing is that they are fundamentally different in how they operate: -
Ignite triggers from nearly every Fire spell. It can be temporarily buffed by 3 talents, permanently buffed by 2 talents and deals twice its damage when you use a Hot Streak.
Icicles triggers from Frostbolt, Flurry and (if talented) Ice Lances on frozen targets, while passively buffing Frozen Orb. It can be permanently buffed by 5 talents.