So I pulled the headset apart today. I had installed the top sealed bearing upside down in the cup. It looks to me like the resulting rocking back and forth "ovalized" the cup, as RetroGrouch was talking about. I flipped the bearing over and the fit is tighter, but alas, the gap isn't something that can be completely solved and it looks like I've finally killed it. It's a testament to how these can last though, it's got thousands and thousands of miles on it and has been installed on at least 4 bikes. And even then is wasn't worn out; it would still be running strong if I hadn't flipped that bearing and ruined it.