How to get avatar arm length correct?

I created an avatar and scaled the arms to be the correct length when I hold my hands out in front of me, that is fully out stretched arms irl is fully out stretched on my avatar. However, if I fully out stretch my arms downward (arms at side), my avatars arms are too short, compared to my irl arms.

Why would this happen and how can I fix it? Is it something to do with the shoulder bone?