Check this screenshot
http://steamcommunity.com/sharedfiles/filedetails/?id=247988840 (All radiants Slo-Lost-Soul (my brother) found were from depth 28 treasure vault. I found 15 and those were from depths 24 to 28 (9 in treasure vault, 3 from depth 27 and 26)
I played a lot of tier 2 danger rooms with Slo-Lost-Soul last days to get some cr and advanced orbs. In all of these days I found a few orbs and Slo-Lost-Soul found many more than me. It also found more sparks... 10 in just a few days when I found my last one a lot of weeks ago!
That may be just bad luck for me and perfect luck for my brother but if there realy is posibile that lower ranks has better chance to find rarity then please fix that OOO because this is unfair :(
What do you think everyone?
http://en.wikipedia.org/wiki/Apophenia