I presume this is because you have added this CSS to the header across all pages, whether they are results of tag or category filters, so the page will always default to TOP and load the full header
I think that if you look at Feverbee, the ‘header’ is only on the home page (and includes a way to hide it) but when you click on the boxes, you load a new template .../tags/... or .../c/... where that header is not loaded
Maybe that helps?