ADE is the viewer used on most readers. ADE also has a lot of rendering quirks when it comes to rendering HTML+CSS. For a partial list of the more egregious ones see
http://calibre.kovidgoyal.net/wiki/ADEQuirks
calibre's EPUB viewer uses the WebKit library (the same engine used by Safari) to render EPUB files, so it will tend to support HTML+CSS much better.
If you can make test cases for each rendering bug in ADE and open tickets, I will try to code workarounds where possible in calibre's EPUB output.