User blog comment:PUNCHURPREGNANT/Gray and Silver Predictions/@comment-96.233.56.21-20140331020647/@comment-7418318-20140404151332

The only one who ever said that ALL the Demon Gates (not Gates of Hell) were from Zeref's book was Cobra.

Leaving aside the matter of his honesty at that moment, he might simply be mistaken. For example someone he knew for sure that 7 of the Gates were from Zeref's books and just assumed that the last two also were. Or maybe one of the Demon Gates told him (or someone from who's thoughts Cobra read it) that they all were from Zeref's Books to intimidate or impress the listener, even though it's not true.

Or maybe the Demon Gates were originally all Demons from the books and Silver replaced their weakest member after proofing himself to be stronger. Cobra might have heard about the founding, but never learned of Silver replacing one of them.