Well it actually has to do with how the loot system seems to work for lost sectors(and why i stopped doing them entirely). as I cataloged 100 master and legend run rewards in the past seasons. What seems to be happening is this: cores and the exotic share the the reward. when you obtain a core as your reward it is taking up/cannceling the possible exotic rewards. Second and the most important of all, is master is not increasing your drop chances at all. All master does is give you an additional roll at rewards while the ratio of exotic drop rate remains the same(this is why receiving 2 cores is very common). third multiple exotics can not roll while cores can(this is important as well because cores obtained negate exotics droping because they count as the activity's reward reducing your odds even further.) I remember it being 60% chance for a core 35% chance for exotic and 15% for nothing. I made a post about this in the past because I found it deceptive to title the reward as "common" when all its doing is giving you is a second roll. When you buy a lottery ticket that has a 1 in 3 chance of winning, your odds of winning dont go up each time you buy one. its still a flat 1 in 3 chance every lottery ticket. So all your doing on master is giving your self two 2 lottery tickets that have a 60% chance to give you a core and a 15% chance to get nothing. or another way to put it is: you roll a 100 sided dice twice, every time you roll 1-75 you failed. This is how gotcha games rng system typically works and i believe its 100% intentionally designed to exploit addictive personality and behavior traits.