Tout l'après-midi, il avait répété dans sa tête

Tout l'après-midi, il avait répété dans sa tête

This answer appears in the writing challemge: "Pre-date stress".

The question was "All afternoon, he'd rehearsed in his head"

Why would the answer not end in "la tête"? Or at least allow both la and sa?

Isn't this a similar case to "il s'est gratté la tête" (He scatched his head) where the body part belongs to the subject of the verb.