It's a longstanding glitch in the way AO3 handles tag sets. The "Sai (DoS)" tag is associated behind the scenes with the canonical Naruto fandom tag, so ends up showing up in there as well as in the Dreaming of Sunshine fandom it was nominated and approved in.

The solution is to make a new tag that doesn't have such an association, which I'll do (along with the doubtless dozens of others in a similar situation across multiple fandoms -- Marvel is particularly notorious for this) once the tag set is closed. (One of the main reasons why I like to make sure all tags contain disambiguations is so that I know where they need to go back to when things like this happen to.)

tl;dr: nothing to worry about, it's just going to cause an annoying bit of extra admin