@The_Look_Of_Disapproval Generally, something is tagged for the kind of magazine or whatever it was in, not what its content is like, as far as demographic goes. This was serialized in Gangan Online. If you take a look at Gangan Online on MAL though, you'll see that a lot of the series aren't even tagged with a demographic at all over there. The ones that are mostly got tagged shounen, but you'll see a few with seinen as well.
Those are probably from people doing their best to pick one based on the fact that the other Gangan magazines and such are either shounen or seinen, but obviously nobody seems to think every single thing on Gangan Online fits with just one or the other so it makes the tag less useful. Honestly some of their stuff would be pretty at home in a shoujo or josei thing too, probably.
This is one of the weaknesses of the tagging system we're using, it's not exactly perfect and there are some things that don't fit the specific definitions some of us want from our tags. I recall there being a magazine that was specifically made for "shoujo manga for men," or something like that. The waters get pretty murky sometimes.
TL;DR: Demographic tags are not an exact science.