That is because my efficiency is clearing the boss as fast as possible. I have the nexus down to 4 minutes, the Mars strike down to 6 minutes, the archon priest down to 5.5 minutes, the devils lair down to 6 minutes, and pho goth down to 7 minutes. Why stop and shoot what I don't have to to up that time for a chance at a legendary engram? I get guaranteed blues the other way that have a chance of being purple.