Procs are not weighted by enemy. For things like on-hit/on-attack, they trigger more quickly against groups because there’s more things hitting you and more things to attack with AoE.
For instance: Lets said you have 25% chance to trigger a skill on-hit (when enemy hit you) and you are surrounded by 10 enemies. Enemies hit you one time per second. In one second you got 10 times (one per enemy) 25% chance to trigger. But when you are fighting a boss usually you are alone against the boss, and lets said boss also hit you one time per second, you only have one time 25% chance to trigger per second. That is why you feel that a skill triggers more often on groups of enemies than bosses.
But i tested it on single (not boss but tough) target and the 25 % pourcent works .
My theory is that proc is considered as a "special move " and booss have some kind of resistance on it.
I reminds me "street fighter " when yu play gains bosses and you try a special move even if youwere in the back of the boss he has auto guard on your move.
Tsunami and few others , it was so obvious on the act 1 end boss and the one at the end of the first wave of the crucible which was very intersting to test on.
This game is fascinating , it looks like a gigantic mathematical benchmark.
I spend more time testing skills and their efficiency than leveling