I know these sort of RNG results are frustrating, but allow me to try to make sense of it:
High Quality + few districts = A deposit with few or no impurities, close to the surface, that is not manpower-intensive to access. However, due its limited land area it is not easy to exploit at a large scale.
Low quality + many districts = A physically very large deposit, but one that is full of impurities, possibly at significant depths below the surface, etc... You can mine a LOT of minerals overall due to its size.
Tthink of a iron ore deposit the size of China, buried under granite mountains, compared to a titanium deposit the size of the Vatican, exposed on the surface.