The rate of heat dissipation into the ambient medium is addressed by the tables.  Comparing the heading from Table 310.15(B)(16) with the heading from Table 310.15(B)(17) shows this difference.

The tables are based on different conditions: Table 310.15(B)(16) is for conductors that are in a raceway or cable or directly buried, where heat is not as easily dissipated; Table 310.15(B)(17) is for conductors in free air where heat is easily dissipated.

This excerpt of the heading from Table 310.15(B)(16) shows the table is for the use of determining the allowable ampacity of conductors in raceways, cable, or directly buried in earth.

This excerpt of the heading from Table 310.15(B)(17) shows the table is for use in determining the allowable ampacity of conductors in free air.