Why are some Champion's Names available as Summoner's Names, while othes aren't?

When I type a Champion's Name in the "Summoner Search" bar, a lvl 6 profile will pop up. But when I type the name of a Champion whose name contains an apostrophe ' , the name is taken by a real player? Like, when I type Kai'Sa, it's not available because Summoner's Names can't contain symbols. But when I type KaiSa, someone has taken it. So that name is actually not banned? I'm just curious :)
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