Slayer instruments

[Ivory Norwind]

I don 't understand...
If i use a slayer instrument i have 10% of chance increase for that monster ( peace, disco and provocation). But i've a little penalty for all others.
If i use an exceptional instrument i have 10% of chance increase for ALL monsters.
It's me that i've not understood well?
It seems a joke

 

[Malkes]

Actually, slayers give you a 20% increased success rate against the type of mobs the slayers covers.

 

[Ivory Norwind]

Ok, thanks