Ok what? She is. She's been learning from the best manipulators in the game, and Arya has been learning in other manners how to be a strong fighter/assassin. Sorry to break it to you, but Arya would never be able to swindle a Vale army to help her, or control the Northern houses like Sansa after Jon left, nor would Sansa stand like a dumbass on a bridge in Braavos when faceless assassins can be anyone.
They've been blatantly clear to give each of the last 4 children strengths/weaknesses
- Sansa politics/manipulation, weak physically/can't fight
- Arya soldier/assassin, no sense for politics
- Jon soldier/war general, meteoric stupidity in politics
- Bran seer/exposition, can't walk/no humanity
It's pretty much an RPG class system but specifically for the Starks.