top reasons:

Spoiler:
jaqen has all that coinage; not arya. jaqen would stare out at the statue of braavos as if he's saying goodbye; not arya. jaqen would throw the coins righthanded; not arya; maisie has trained to be lefthanded too. arya/jaqen was loud and obvious and clumsy. jaqen/arya was walking like a highborn, dressed as a an obvious westerosi. jaqen still owes arya his own life; remember how she named him.