Having said that, it does work at certain browser percentages on certain browsers just not on all the same, so its potentially more than just the css itself.
The way your test site is set up the issue I have will never occur. On your site the header padding is horizontally centering the menu and header image and the menu heights are consistent between the main menu and the sticky menu where as mine aren’t and why I need to add the 10px margin which at certain browser percentages appears to vanish causing the menu to move up 10px.
Garth