I think Jace was right about the dark coils. I compare the old ballast (from this fixture) to my new 100 watt HPS M-250 R2 ballast.. The coils on the 250 were MUCH darker than on the 100w HPS ballast, and both looked like they were made from the same stuff.

So I guess this ballast was at the end of its life. Especially it being in a small fixture.