You're forgetting one crucial fact.
Xur doesn't carry ALL the exotics. He has a certain pool to pick from.
Meaning that 1 in 6 is actually 1 in 2 or perhaps 3.
Suddenly that chance is looking pretty slimmer isn't it?
This is another double RNG at work. The first roll determines which slot Warlock armor is selected. Then it selects from a very limited pool to fill with another RNG.
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Doesn't make this particular example of Math correct. The Warlock armor slot will never carry an Exotic weapon. If anything that proves my point in that Xur only carries a limited pool. Because he doesn't carry ALL the exotic weapons. Unlike this Math which claims he carries ALL the Exotic Warlock chest armor.