It's easier for others to detect your email is AI written in your style than for you to detect it.

You'd think that you'd be the best judge of your own style, and to some superficial level you are.

But you recognize the point you're trying to make from inside and iterate until the AI one sounds enough like you making the point.

But others hear the point from outside, where the way it sounds is most of the payload.

Kind of similar to how if you show a reflected image of you to your friends it looks totally normal to you… and totally "off" to them.