You have your tinfoil hat on too tight, it's suffocating your pea sized brain.
Just because you "know a guy" or you have shit luck luck you think that's scientific evidence enough to formulate a drop percent? Lol ok.
Myself and many of my friends have these decrypt from engrams fairly often. So by your idiot logic and scientific methods I conclude that you are wrong.
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So you marked that you forgot or got them from Xur?
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I didn't mark anything because I've gotten them from everything except Xur. It's pointless to ask where people got them from because they have an equal chance of dropping from EVERYTHING you listed.