The reason why Jinx's ult got buffed is simple
Look at Ashe's and compare
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For Ashe, you get an in-lane insta-fight in your favor, a long ranged savior sent from who-knows-how-far-away, or even a strong teamfight initiator. That ult's stun is insane, and with enough travel time, the person hit is almost guaranteed to die.
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For Jinx, all you get is damage. It just picks survivors. That's it. Whether it's from 2 lanes away or it's simply used as a mid ranged finisher, it only does damage, and there's no other use for it.
Basically, Jinx's ult is outclassed by Ashe's in almost every way, so in order to further keep utility marksmen from crowding out the carries and damage dealers with the upcoming patch, they're making it so Jinx's ult is better at what it does: Finishing off survivors from a distance. Not saying Jinx is weak. We're just saying her ult is the most "meh" part of her kit. If it does too much damage or something, they might reduce the ratio to like 125% or something if I had to guess. They still get one more patch before worlds, right?
Anyway, I just wanted to make a thread to put that in perspective. A lot of people have seemed confused about it...but it's kind of obvious.