If I may; I believe the Random Number system has a certain chance for any particular type engram to drop, no matter what event you are in. Whether it drops as a reward or from a slain enemy, there is a random chance of each type of engram dropping. I just completed my Thorn bounty (thanks to my clan who helped me through the Xyor strike), which is a level 26 epic strike, and guess what my rewards were, other than the bounty itself? A green helmet and scout rifle, as I recall. 2 blue engrams dropped during the strike itself, and a few uncommon items from dead foes. Your reward will not always be of a guaranteed level. I have not gone through many raids, but I have done a few nightfalls by now, and my rewards are not always legendary. I have never had a purple engram drop during the event.