Morning Oliver;
Did a quick test with Graphene on my test-blog, and the admin-bar sits nicely at the top as it should.
No idea what specifically, but you probably have a plugin that has conflicting directives once optimized. Best approach would be to either selectively disable plugins to see which one is to blame or to use your browser's developer toolbar to see what styles are applied on the admin-bar, see which one is forcing it at the bottom and going from there.
hope this helps,
frank