That occurred to me as well. The only answer I can come up with would be statistical, which would require quite a number of observers fairly near other observers. Then there are questions about certain areas of the country having a different age-profile and therefore, presumably, different quality of eyesight. Trying to allow for things like that would be a little bit of a nightmare (a 'nightfoal', perhaps?)

More likely, they won't bother and will just report the raw facts, however potentially misleading they are.