Looks like the navigation through a gallery from within the attachment pages only works (as i would suppose it to) if the attachments are linked(/uploaded) to that particular post/page.
Let's say i have uploaded ten images on an older post before.
Now i make a new post and choose three of those images from the media library, create a new gallery with those and put them in that new post. Now the thumbnails will show up correctly, a click on an image will take me to the attachment page. But then when you click through that gallery with the Next / Previous Links it will take me through all ten images instead of only those three.
Also if you upload two separate galleries with 5 images each to the same post it will cycle through all ten.
I'm not sure if this is a bug or if i misunderstand how the gallery navigation is supposed to work but it would make more sense to me if the attachment pages cycles only through the images in one gallery...?
(btw: tried with 3.5-RC1)