User blog comment:Silicon Soldier/Xûr's Inventory: July 31, 2015 - August 2, 2015/@comment-4957813-20150801155151/@comment-25916964-20150802071441

Not quite 5 weeks in a row. But it did go 4 weeks of helmets, then Chest Armor, and now back to Helmets already. Wouldn't happen as often if there were more than 3 exotic engrams. 3 since there is no exotic warlock leg armor.

Funily enough, looking at the Visit History, these long spans of the same engram are rather common with Chest Armor and Helmets. Haven't looked at the exact stats, but both appear to outnumber exotic Gauntlet engrams easily.