My guess is they ordered a Drop lens polycarbonate, maybe on accident, and since Xcel energy favors FCO before they put it up they put a flat glass in there.
Yep, for some reason AEL will let you order a polycarbonate refractor up to 250w HPS/MH on the 115 and 400w HPS on the 315! I'm guessing it would last about 3 months or so at the most before disintegrating on the 400w HPS, and from what I have observed it would probably last about a year on 250w. What I think happened here is the original polycarbonate refractor became so brown and brittle that it broke apart and so they put an FCO glass in it.