Ranger buff: I need a clarification on how it is supposed to proc.

So the Ranger origin states the following:

Rangers have a chance to double their attack speed for the next 3s.

• (2): 25% Chance to Double Attack Speed
• (4): 65% Chance to Double Attack Speed

My question is this: Does this mean each Ranger have a chance to proc the buff individually on themselves (1 Ranger procs it for himself), or they collectively have a chance to proc it on all of them, at the same time (One Ranger procs the buff for all of them)?

I'm asking this because, a lot of times, the buff procs on all of them at once, suggesting that the buff procs for all of them at the same time, and sometimes the buff just procs at one unit at a time, suggesting that the proc chance works individually on each unit rather than collectively.

Can anyone clarify this for me, please?