You would need to have thousands of engrams to get a proper gauge on actual drop rates and probably input from many players. Considering you're still working with even percents in RoI, I doubt that is there. I for one have gotten a shitload of special weapons since the release of RoI. It makes sense that armour drops more than weapons considering there are 4 armour engrams and only 3 heavy (which actually works out to your TTK percentage). From this, it's still perfectly possible that each of the different exotic engrams has an equal drop rate which I'd say is more than likely. The way this game has worked with its loot tables, I'd say it's perfectly likely that it's totally RNG that selects equally from among each of the exotic engrams in the table.

Your statistics once again prove that it all is totally random. The RNG works. Exactly the same now as it did before. Now- to make it even more interesting, did you happen to keep track of total number of drops for each? Total number of 3OC needed per drop? I'm betting those results are totally random as well.