In the update article there's no mention of this change, but the cone of fire in the Volley preview video was noticably a different shape. There's all this talk of making Ashe the utility adc, but her ability to slow over a wide area seems hindered compared to before. Ashe's W also already had silly poke range to begin with, now it's even further?
I feel this is because of the new per level mechanic of adding arrows to the skill. Although you cannot be hit by more than 1 arrow, allowing such a wide cone and even more arrows would make the skill even more easy to hit. I think it's more to do with narrowing the ability while keeping the power about the same.
It was mentioned, in fact.
It was to help around with her laning, due to how her live arrow's being spread out causing a lot of poke to get through that normally wouldn't.
I'm pretty sure it still has the same max width at max range they just increased the range which makes the width smaller when its close to her. From my understanding of their post this was to giver her the same team fight cc potential while allowing a well positioned tank to stop that cc from reaching the backline.
