User blog comment:Fire Eater/Differences between the manga and the anime?/@comment-1037137-20180629192749/@comment-1327106-20180629202836

The Survey Corps got sloppy? ¯\_(ツ)_/¯

I do think they were celebrating prematurely (who does that when you're still flying over enemy territory?) but there's no in universe explanation for it. My feeling is that the airship itself was being manned by a skeleton crew though, so it's possible there was no one in the turret initially because whoever would have been the gunner was actually down below fighting, but Armin could have taken a seat later after he finished with his Colossus business.