As far as I'm aware, I was the first one to do this, which was only on the Index/Railgun character list. It was then somewhat copied to the others by Daniel21. For the Index list at the time characters fell into 3 categories: those with a ton of images (were shown in the anime, tag bolded), those with very few images (likely not shown in the anime, tag left normal), and those that only exist as a tag because the character appears in a small image on a chart (in short don't really have any images of but the tag exists, tag italicized). At minimum I thought being able to distinguish that the tag didn't really have any images, but that the tag did exist due to a chart or something would be useful when looking at the list. The divide between low pop tags and high pop tags wasn't really necessary, and the divide was only setup in relationship to population of index character tags at the time, not something that I thought should/would be applied to other character lists.