Hi Chris,
I just received an explanation from our developer: Area Unit affects the tools that is with "editing" purpose, but Measurement tool does not carry editing purpose. Therefore, it gets the information from Numbers without Units part.
I can see that it is confusing, Unfortunately, it's a legacy setting that is coming from previous versions, and it would be a big project for changing the way how it behaves.
Thank you for your understanding! If you have any further questions, feel free to ask me again!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT