There is no standards policing body checking company claims about wattage. Therefore, there's a lot of corporate misinformation and flat out lies. Some claim peak wattage when RMS is a much more useful number, and typically half of the peak rating. Even when reporting both Peak and RMS, there are ways to fudge the numbers, and fudging is the norm in the SS industry.
Before tube amps, tubes were used in everything, like scientific instruments and military gear, so tube specs were strictly defined, and actual output known without question. The tube types haven't changed much since then, and the correct specs persist.
For some reason I don't understand or care about, when SS tech came into use, companies found wiggle room and abused it.
So what a watt is, is strictly defined. How shady companies measure and report wattage, is not.