[Al Skierkiewicz]

James,
Wire specs for NEC are determined by temperature rise for the insulation and by voltage drop at specified current over a specified length. In the case of the flat wire, certain signals have a hard time propagating down flat wire. For audio (and other AC currents), the shunt capacitance also becomes a factor as the max 80' length spec alludes. The 20 amp spec is likely because this product is intended to be painted over. At more than 20 amps continuous, the paint would likely start to bubble.
As for NEC current specs, the test on a single wire size is how much current can the wire transmit before the voltage drop at the end of a standard length exceeds a certain percentage. The current/voltage standards used for aircraft instruments is also used and more stringent.