I'm not too familiar with it myself, but if I had to hazard a guess it's because setting an element to display:fixed breaks it out into its own root stacking context[0], and browser zoom affects every stacking context[1].

Would love to hear from someone who's actually familiar with what's going on though.

[0] http://updates.html5rocks.com/2012/09/Stacking-Changes-Comin...

[1] https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Under...