I'm not an engineer.
That being said, here's my 2 cents worth. I think the 12/12 brace is used because it's balanced. Imagine, as an extreme example, a 2/12 brace. A lateral force which puts the brace in compression would have a mechanical advantage in pushing the post away rather than directing some of the load down the post. That is, the force would be directed more horizontally. If the brace were set at 12/2,the brace would do almost nothing for horizontal forces.
Another place to look at members which work to maintain an angle would be rafter ties. Of course you'd want a collar tie to be level and it's intuitive that the angles are equal at each end thus creating mechanical balance. I would argue that if the roof had dissimilar pitches, the most effective collar tie would be something other than level. Dissimilar rafter lengths or anything that makes the roof geometry unbalanced would have a similar effect.
It's all 'bout balance.
None of this is to say that a brace HAS to be 12/12. Variants are seen all the time. As with any frame design question, let an engineer check it if there's any doubt.