maximum cooling watts and maximum heating amps are rated the same at 7.4 amps so this is for the heat pump only running in either mode.. Your unit also has electric heat backup but specs don't list the wattage rating of the electric heat. I'm assuming it's a 1500 watt heating element, but what ever the wattage is the heat pump and electric backup may both run at the same time periodically taking you over the 20 amp breaker threshold so 30 amp is the next one in line.

Breakers and the wires feeding a unit are usually supplied at 125% over the maximum current draw. The KCM18 model is just an Air conditioner only and running current is 9.6 watts, and only requires a 220v 15 amp circuit, so I'm 99% sure it's because your unit has the electric heating element included.

The user guide advises that heat pump models with electric/resistance backup* automatically switch to resistance at 37°F outdoor temp (back to heat pump at 40°F). A 1,500 watt element wouldn't be able to supply the equivalent full rated heat capacity. Model KEM18A34A is the sibling non-heat pump model, rated 19.5 amps, 4,200 watts for heating. I'm guessing the resistance heating amps/watts isn't for some reason isn't stated for KHM18A34A on the specifications document.

*Model KSM10 is 115v and doesn't have resistance backup per the user guide and specifications document.

Note also that the KHM18A34A is equipped with a cord and plug for a NEMA 6-30R 30 Amp 250-Volt receptacle.