In both Borg incursions into Federation space, Admirals were in command of ships that flagged larger battle groups.
What evidence do we have for this? In neither instance was the Admiral seen sitting in the center chair of the starship, nor was there proof that a CO of Captain rank would have been absent.
It is quite possible and even likely that the Admirals were commanding from abroad the ships, not commanding the ships themselves.
I'm sure it's open to interpretation, but it sure looks to me like Admiral Hanson is in a center seat at Wolf 359: http://tng.trekcore.com/gallery/thum...lbum=74&page=6
. Is that a bridge railing behind him?