Uh maybe they look at the voter's IP address or they just didn't think of it lol... Is it unlawful to submit multiple votes (not that I would do ittttt ٩(๑❛ᴗ❛๑)۶)
I translated a bit of the text at the top of the page and I think it said that "even if you vote multiple times, it will not be reflected in the ranking"
https://www.anime-japan.jp/main/ajranking/
by the way, how is it counted exactly? people can vote countless time? because I tried votinh twice and both are successful