What I mean is that, for example a Lightning merc has a frequency of 60 for Charged Bolt, 30 for Lightning and 10 for normal attack. "Frequency" governs what action the AI will choose when the merc targets and attacks an enemy. Thus, 10+20+60 = 100 total frequency, and 10/100 = 10% chance for normal attack.
A Cold merc has:
1000 frequency for Frozen Armor
240 frequency for Ice Bolt
60 frequency for Glacial Spike
10 frequency for Attack
That's why you'll always see the merc cast Frozen Armor as soon as you leave town, and as soon as it wears off. From the remainder, 10/310 = 3.226% chance for normal attack to occur.
It's not accurate, but it'll blow your freaking mind...
So yeah putting items with "Chance to cast on Striking/Attack" on any Act 3 merc is a bad idea. You're better off putting a Lawbringer on an Act 5 merc.
Or, even better, a Reaper's Toll on an Act 2 merc. You get a higher chance for Decrepify, and Reaper's Toll is a better weapon than Lawbringer as well.