It's Miyu!verse influenced. The corruption from the overflowing amounts of bro and sis con taint the very concepts associated with the worlds they reside upon.
Plus, we totes kidnapped Sakura and made her busty Rin with a kind attitude who liked us anyway but is now indebted to us for saving her life (even if Shirou won't realize she thinks that). Then you go summon grateful assassin, who appears to have an astounding physical resemblance to Worm!Sakura who looks upon us as the greatest thing to have ever possibly happened to her because we arent immediately dead. There is no way that the people who voted Serenity weren't influenced by the desire to have their cake and it too
Edit: I sound like an incredible troll but nothing there is really wrong explicitly with what I am saying except me joshing about the cake and the waifu. If you tilt your head a bit and look at it from a 5 degree angle... it does not paint a good light for Shirou here. Not at all.