Some manufacturers will rate the power at only one frequency, I believe 1 kHz, instead of the full range of 20Hz to 20kHz. The 1kHz rating might be 100 watts which sounds better than the full range rating of 90 watts. Even the same company, like Yamaha, will sell two different product lines, which are actually the exact same unit under different names. The mass market product line will use the 1kHz rating with the higher power rating (looks good to the uneducated buyer) and then use the full range rating in the higher end line. The specs will usually state how the power level was determined, so you can usually tell by checking them out. Such as "100 watts at 1kHz" or 90 watts 20Hz to 20kHz or using teams such as "full spectrum" or something like that.
It has been quite a while since I have looked at this kind of info, so I am basing it on a very fuzzy memory, so if I mistated any of this, please feel free to correct or clarify it.
