Why do some Google results show the
http prefix in the green URL and some don’t?
Google does not display
http://, it does however display
https:// and does this to indicate to users that the site uses SSL. Sites that do not force SSL will display with no protocol.
Google is pushing for SSL to become the standard while it has users security in mind but it is also my opinion that they have an agenda behind this... because when all search engines, sites and users start using SSL finding out your keywords will become impossible and the only way to obtain that information will be to pay for it.. i.e Google.