Typographical errors are a facked of life ;-)
We all do it so please be kind to each other's mistakes.
Piett isn't Vader, the Tydirium had an older code so he was a bit suspicious about it, he freely admits he would have let them pass anyway but he picks up on Vader's hesitation and speaks up.
Vader on the other hand knows Luke is there but wants him to land without making a fuss about it (Vader doesn't seem to be the type to chin-wag with his underlings on his coffee break).