Yep, if i did the right maths... You have 21 chances each week to get an exotic weapon 3 nightfalls, 3 ir yut, 3 crota normal, 3 Hard, 3 vog normal, 3 Hard, 3 gorgon chests, weapons like fatebringer, word of crota etc etc, you only have 3 chances every week, but no one complains about those lool. So for me, no reason for xur to sell weapons

Earned as in login in daily for a -blam!-ing participation award, its completely random, you cant earn anything in this game since you cant really farm for a specific item