Actually I have a pretty good idea since I have a vault full with legendary and exotics to where I just dismantle them now and I've yet to buy anything beyond a guantlet with Vanguard Marks. One person getting an exotic off an end game roll is worse odds than getting a Legendary off a Mob drop during the Strike. The end game roll is not better odds than Mob drops for Legendaries. It's just another way to get them which is why if you want to maximize your chances you work to complete the Strikes quickly but you don't just ignore half the enemies.