G'day
The introduction to t book I have called "Visions of Wonder" explains it this way:
'Myths and faery tales belong to the category of high fantasy, while such stories as tall tales, animal fables, and Thurber-tye farces are included in the category of low fantasy. In the letter group the setting is the real, or primary, world, and the rationally unexplainable phenomena have no assignable cause; they are simply anomalies. In high fantasy, by contrast, the setting is a secondary world, set apart from our familiar one, in which rationally unexplainable elements find a proper explanation in terms of magical 9faery tales) or supernatural (myths) causality.'
Regards,
Agback