A week after the attack, Ambassador Rice went on 5 Sunday talkshows, andsaid there was no evidence that this was anything other than a spontaneous protest to a movie.
We now know that hours after the attack, a known terroriet group claimed responsibility, and we know that the administration was aware of that claim.
Libya Attack Emails: White House Told Of Militant Claim 2 Hours After Benghazi attack
Likely conclusion: the Obama administration knew they screwed up by denying Stevens the security he asked for (and provided lots of supporting evidence for why he needed it). Rather than admit their mistake (which Obama is not known for doing), they concocted a story to make it look like this was something other than what it was, in the hopes that it would make them look less incompetent.
Hope and change.