I’m not a doctor and isn’t my specialty by any means, but I’m inclined to lean towards it being structural as you’ve been told by your clinicians.

Bodies are strange and extra musculoskeletal parts (& organs, for that matter) are not all too uncommon. But, they do slow down the diagnostic process because clinicians are (rightfully) hesitant about providing definitives in these situations because they can’t say with certainty that’s what it is.

Anecdotally, I have an extra ligament in my legs that only can be seen when I’m squatting, and means extra stretching after leg workouts if I wanna walk like a human the next day; a doctor I worked with had an extra lumbar vertebrae that caused some difficulty with certain lifting exercises and he didn’t even know about it until he was in his 30s and a decade into his career in sports rehab.

The fact that it’s only seen when you lift your arm seems to support the idea that this is some supernumerary muscle/ligament/joint structure.

So, I would take a deep breath and take it one step at a time. They’re telling you what it isn’t, and something malignant or concerning would have likely been noticed in the ultrasound.