
AP auto-attack tower damage modifier is using season 8 numbers instead of season 1-3
In season 8, Riot changed the AP tower damage to be 60% of AP. In the first three seasons, it was 40%. Tristana has 263 AP, so with the tower MR she should deal 75 magic damage, but she deals 111, which is what she would do with the modern numbers.
I knew that towers felt too weak for some reason while playing AP champs. This is most likely why.