An underfed variety.
It could be a specific cultivar or just a seedling. Loquat seeds are notoriously easy to sprout -- often from a nearby tree or a feasting bird, to the point of being a nuisance.
Even if it was fed a robust diet, it would still be difficult to identify by picture or even taste. One person who is somewhat of an expert in the tastes of Loquats is Jim Neitzel of the San Diego CRFG chapter.