UL tests 15 and 20 amp receptacles to the same standards. This is done because the NEC allows 15 amp patterns to be used on 20 amp circuits.
A 20 amp receptacle is tested, assuming a 20 amp circuit. A 30 amp pattern assumes a 30 amp load, etc. So it does matter that you use the right plug pattern for the circuit.
The NEC allows the exception for 15 amp pattern receptacles, because a 20 amp circuit with multiple outlets is, almost by definition, a convenience or general purpose circuit. There is really no way of knowing what loads will be applied, just a near-certainty that they will be numerous, and much less than the rating of the circuit.
The same reasoning cannot apply to a circuit dedicated to a specific appliance. In that circumstance, the circuit - as a whole, and not just the plug pattern- is based upon the specific needs of the appliance.
