The "doesn't add up to 6.5gb" is because storage is shared with other services and doesn't pertain to just Google Drive.

I agree with Google, that it is more important to show how much overall storage you have left, than how much is being used in this particular service, but there needs to be an overview that can show where storage is used. And clicking around on Google Drive it appears there isn't such an overview.

So yes, bad UI, but for different reason than what grumpy website says.