I noticed the same thing. Maybe I got a little unlucky with my testing but if anything the % chance may be even lower than 10%. I knew the bonus was pretty terrible on BR so I waited for a while before picking up the pieces, and only did so because as individual pieces they were better than what I had equipped in those slots before (I picked up the helm and shoulders, to replace the Hytbold shoulders and some random skirm helm).
It seems both tank sets seem to be set up like that: terrible 2-set bonus and good 4-set bonus. I guess on one hand it's a good thing, because it'll allow me to mix and match between 4 of one set and 2 random pieces from anywhere based on their stats. I do like having three set bonuses though, so it's a bit unfortunate.
I will probably be going 4 Erebor and 2 misc whenever I finish Flight T2.
Steam: Sneakeh Snake[/CENTER]