Posture and mannerisms provide subconscious gender hints to a person looking at us. It's not just facial structure or body form. If you were to ASK someone why they gendered us the way they do, they would likely make up a tale to explain what their preoptic cortex did, as gender recognition in a casual encounter is not a conscious process in the vast majority of people.
Head and mouth motion, gait, and posture are all cues to this recognition process, along with static physical appearance. (I've been clocked from behind, although not recently, I think 🤔 ).
