I believe this is true and there is some kind of cycle. I can't get a suros. But once I get a weapon its like that same week it drops constantly. My first exotic was thunderlord. Dropped two more times that week. I didn't get an icebreaker until I bought it from xur like 2 months ago or whenever he sold it. After that literally 5 more dropped since. Same pattern. 2 red deaths, 2 ghorn, 7 vex, yes 7. No suros. Oh yea and with my 4 no land beyond. This week finally a new weapon and I get multi tool in nightfall, vog chest right after and I get my second multi tool. Same pattern for armor pieces that drop. So there has to be some type of system where certain exotics are in your specific pool for chances to drop.