I'm playing a devil's advocate here, but it doesn't have to be designed that way, just a product of systems coliding - you have a way for the game to measure how long has it been since your last lego drop, which they said takes into accout a lot of variables, then you have the loot 2.0 making sure items for your class drop ~85% of the time and last but not least, Kadala drops both lvl 70 and lvl 61 versions of items, which would make her work a bit differently compared to your average mob. I can also imagine they spent some time to make sure the loot isn't too random, because they need to give you items to keep you going.
Let's say they bias the loot table toward your weakest slots, doesn't have to be much, just enough to make an upgrade likely enough and the way the item rolls is still random, so you get your fair share of shitty rings.