As you're new aperture turret came in the mail, you were expecting it to be a normal one, just maybe. Nope. You opened it up to get Turret-android instead. Oh and she's got a gun.

Sentry-Chan: "Were you expecting a boring old regular Aperture Turret? Nope! You got me instead, Turret Girl! But you can call me Sentry-Chan! I've got the same things as a regular aperture turret, but now I'm also an android!"