Safari appears to retain login information, site preferences, or cookies even after clearing history, cache, cookies, etc. I even tried Reset Safari..., which I understand should restore Safari to it's original state retaining no history, etc. Plus, I set the default page to be an Empty Page. Despite all it appears that at least StackExchange still recognizes me and is able to re-login automatically with my credentials.
Give it a try yourself. Login to a stackexchange site... say superuser.com, then without clicking the logout button, visit another site. Now do all of the above to Safari, Empty Cache... Reset Safari... go into preferences -> Security and clear all cookies, then adjust preferences to startup on an Empty Page, heck, shutdown Safari and restart your computer. Now, relaunch Safari. It should startup on an empty page. Now visit superuser.com... wait for it... tadda! Superuser.com recognizes you and logs you in!! This is unexpected behavior and different from any other browser. One would expect that clearing cookies would have been enough to require that you re-enter login information.
So, what's up here? Doesn't this hint that despite clearing privacy information, sites can still recognize thus "follow" you by some other means... or that Safari simply isn't doing what you ask it to do, clear cookies and cache.