ampacity also differs based on the insulation type, or lack there of for bare and overhead conductors, type of raceways the conductors are installed in, no.of conductors in an assembly (a multiconductor cable ) vs. a single conductor etc.
Not sure where you are, NEC has quiet a number of tables for ampacities for the same reason. Ultimately it is related to heat dissipation to keep the temperature of the conductor within acceptable limits