It's coming from loosened tolerances. Higher/lower temperatures may mean higher loss. I'm assuming there's a bit more tolerance in the alignment of the up opt

It's coming from loosened tolerances. Higher/lower temperatures may mean higher loss. I'm assuming there's a bit more tolerance in the alignment of the up optics/connectors as well. So, if you get lucky it might well be 5db@2km, but it's not guaranteed/required by spec.