Normal -> Magic -> Rare -> Unique Monsters

Normal - Items may be up to the level of the area.
Magic - Items may be up to +1 of the level of the area.
Rare - Items may be up to +2 of the level of the area.
Unique - Items may be up to +2 of the level of the area.

...

Looking at the progression, why don't unique monsters drop items +3 of the level of the area just wondering? It goes 0-1-2-2, which seems odd to me.

Was it tested out and found to farmable or something?

Chuck Norris will be added to the game at a later date, which will drop +3. Unfortunately, Chuck is not farmable.

I guess its because most unique monsters are quest related and have +quantity bonus, so you would still get better rewards though their average levels are not higher than rares.