"It's not the heat...
     It's the Humidity"

The truth is, it's probably both.

Why is it that people only say that when it's hot and sticky and their Air Conditioning is "broken"?

The reason is that when your Air Conditioner is "broken", it does not remove the moisture from the air. Moist air is cold in the winter and hot in the summer. The secret to a comfortable home is removing that moisture from the air. Your Air Conditioner usually becomes inefficient for reasons that could be prevented! NOT replacing the filters regularly, NOT having the evaporator and condenser coils cleaned or NOT having enough refrigerant in the system.