I think the 100% is relative to the thumbnail size as set under Preferences > Explorer > Thumbnails. The standard (100%) is either 320px wide (Hi-quality) or 160px wide (standard quality. So you can zoom higher, or compress lower.
See
https://jalbum.net/help/en/jAlbum/Preferences#Thumbnails
More, I have a photo with an embedded thumbnail of 256px width. If I set jAlbums zoom to 100% and copy that screen image and get info on the resulting image it reports 160px (on a Mac so it is really 320px). That is the size I have set under the preferences above.