Different numbers of strands have different amounts of copper for a given diameter. The resistivity of pure copper is 1.7x10-8 ohm-m. The formula for resistance of a conductor is

R= rL/A where I am using "r" for resistivity, L is the length of the conductor and A is the cross sectional area of the conductor. Thus over a 60' (18.3 m) run a 3/8" diameter solid conductor will have a voltage drop of 1.96 volts. Stranded wire will have a larger voltage drop. Similarly a 13mm diameter solid conductor will have a voltage drop of 1.05 volts for the amperage you noted. Basic physics 101