You mean the sources who were on the ground in Benghazi"? Do you doubt them?
You mean an unnamed source who was supposidly on the ground in Benghazi?
If one doubts information from an unnamed source then it stands to reason that one would doubt such an unnamed source was "on the ground" at such-and-such a location.
About the spcific contradiction between Petraeus and this unnamed source, it's worth noting that the unnamed source refers to the CIA chain of command which would include people outside the CIA, while Petraeus only refers to people inside the CIA.