If Safari finds a very close match to your search query or a recently visited page, it will show that at the top of the list as a Top Hit.
Safari 6 also changes the way a web address displays in the address bar. The "http://" is no longer shown which makes sense since you haven't needed to enter the "http://" part of a web address in years. Also the part of the web address after the first "/" is now shown in grey text. Most people don't bother looking at the full web address anyway so the grey text makes it easier to ignore.
When you visit a secure website, Safari lets you know right at the beginning of the web address so it is more obvious that you are on a secure web page.
Secure websites begin with "https://" (note the extra s). Safari has always shown a padlock icon when you visited a secure website, but it was displayed at the top right of the Safari window. Now the padlock icon shows to the left of the web address. (more about https:// here.)
Secure websites with certificates show the certificate name in green at the left of the web address as well. (more about certificates here.)