Nope. My theory is just that some weapons have a lot less chance of dropping than others (same with being included in Xur's loot table, if he IS random). Bungie alter this % chance every so often to even the playing field, as it were. Also I think that each week, there is a loot table of exotics that can be found through deathsinger/atheon/crota/VoG chest & maybe nightfalls (though I think nightfalls is a separate one). We've had so many people get the same exotic in the same week too many times for this theory to not be viable.