User blog comment:FairyRaven/Fairy Tail Chapter 362: Natsu vs. Jackal, Review and Commentary/@comment-119.95.232.64-20131129112258/@comment-7418318-20131129115217

Tartarus was extremely secretive, so no one would have known how strong they really were.

Then there's the matter of that assessment being seven years out of date, which is plenty of time for the balance of power to shift.

And don't forget that GH had an entire army of minor mages.