Alright, my opinion that I'm weighing in is that Volstagg is mostly an idiotic braggart (which he is. He's my favorite of the Warriors Three and is my favorite Asgardian period. He's a coward, an idiot and a braggart and never should be taken at his word).
What I find confusing is the second scan. The only thing we have to go on is Galactus' eyes and his hand, which really doesn't actually show what's going through his head at the time.
My opinion on the matter, which is just my opinion and can not be backed up by un-inferred on-panel evidence, is that during the second scan Galactus is in telepathic contact with the Silver Surfer and he leaves the battlefield once he learns that what he has sought after is no longer there.
We can't take Volstagg's word as the gospel truth and I personally would see Odin's line about "braining" as him just trying to save face. Because honestly, regardless of who you think would win or who is more powerful, he definitely came out looking way worse from that confrontation than Galactus did.
But that's just my opinion as a previously impartial third party observer. Feel free to rip it apart or flame it or whatever will definitely end up happening because my opinion runs counter to the idea of Galactus being a coward.