Not sure about the new bucks, but it's always meant an amp rated at a particular rms that produces a considerable amount more in reality. This allows competitors to enter a certain class, but have more power than most others. In essence "cheating." RF used to be really notorious for their underrated amps. they still underrate by the widest margin on avg that i've seen.
Orion and Earthquake used to put out some of the best cheater amps. Some of there older amps would be rated in the 300 watt class and be pushing almost 3000 watts.