Well if ALL Agonys can proc it, you run into the problem that it becomes crazy powerful on multidot fights. Now, I like to think that the devs have at least the smarts to avoid compounding the "multidot problem" that way. For example, on a three target council fight, you'd multiply up that proc chance to 32% unless there's a diminishing return mechanism like there is for shards. I really doubt they'd want such a high chance, particularly as it would mean affliction would be incredibly powerful on fights where we're already incredibly strong anyway
To my mind, it means "the Agony on this target has an 8% chance of extending UA on this target"
Anything lower, let alone 2%, just meant that the 2-bonus was worthless on a single target fight, where aff is comparatively weak. Limiting the proc to the same target means you can have a decent chance and avoids the multi-target multiplication issue.
I'd rather have it decent for single target combined with not multiplying up on council fights, than having it worthless shit on single target because it multiplie sup on a council fight.
It would also mean it would work fine with Contagion because it turns UA more into a maintenance dot than a shard dump - given that the proc is wasted if UA is not running, you'd want to keep UA uptimes higher. Contagion is a singe target talent really, since AC's value rises the more targets you have so the 2-bonus would work hand in had with Contagion and both wouldbe single target enhancers, where aff's primary weakness lays.
We certainly do not need bonuses that increase multidot power because we are already so strong there, and it would invariable result in a generalised tuning down, which our singe target does not need at all. It wold make aff more rounded (though I disliek the idea of tier bonus crutches as it hurts peopel without tier armour)
As you say it does not indicate whether the proc will extend ALL the UA's running or one of them. However, as I stated, given that the proc is wasted if you happen to not have UA up at all, this encourages use of UA as a maintenance dot rather than having "dumps"
Even now we are 'encouraged' to have a steady use of UA rather than the old style of doing dumps. Particularly as shard generation will be much lower, the T21 does not increase shard procs at all, whereas the T19 and even T20 do.
So you would wish to husband your UA's more.