The same as with any data you'd collect about anything -- it's part of a larger ecosystem, and no reasonable person expects one number to be the ultimate fount of all wisdom.

I can recall situations where some of my reports were unhappy, which I knew about, but where the _magnitude_ of their unhappiness didn't come across, because for whatever reason, that's a thing people will often hide in conversation, at least for a while. Similarly, anybody who sees patients will admit to getting very different readings from different types of solicitations about their well-being. They become more useful in aggregate, and with experience.

Also, nothing prevents you from including a free text box to add context to a "happiness" rating.