2006 Scott CR1 SL - FSA 'ACB' integrated headset

Every once in a while the headset squeaks when I rotate the handlebars. Loosening the preload bolt and the stem bolts and then retightening them seems to fix it. I think I have it narrowed down to how tight I make the pre-load bolt; too tight and the sqeak appears. Back it off and it's ok.

What I'm not sure I understand is why this squeak appears over time. I could see how it could become looser, but not tighter?