User blog comment:Hitsuji Mamoru/Servant Creation Room 2 : Guess of the Who/@comment-27247829-20170327060453/@comment-4802891-20170328061055

Considering her intro line Valeth, Liu Bei is surprised to be summoned in the Avenger class herself. In a headcanon kind of situation, I can imagine this to be a singularity-specific situation like Bedivere or Moriarty, or an unorthodox summoning ritual to forcefully summon the Spirit in a Class not suited for it.

She is only Avenger because of the grief at the end of her life and some bad decisions dictated by it. As you said, this isn't enough for normal circumstances, but this is FGO, the game where holding a watergun is enough to make you an Archer...