I've been wondering that myself.
Could it be that the buff carries over? Meaning if you have the buff but don't get one that day, you are able to get two the next day?
I downloaded the game when they gave out a free copy, recently installed it and bought the dlc and hit level 50 today. Along with that, every single time I start another mission or strike or any kind of playlist really I find at least one if not two prime engrams, so I think maybe it stores up since they launched? But I'm not sure o.o
That's possible. I've heard that this is highly likely. But I also have been playing every day since Forsaken launched, and i'm almost sure that I got at least 1 or more per day.
Some friends got multiple today and were also positive they got at least one every day before. For me the buff went away right after the first one and I didn't get any more drops.