Search's documents tab just seems to suggest search in file explorer as the best match

It seems every time I look for document in search and select the documents tab I am told that the Best Match is searching in file explorer!

Is this because

a) I am unlucky with the field I look for

b) something is broken (I am not sure I have recently seen a file returned)

c) the feature is just not very good

Seaches for apps and web work OK.