it isn't just glow engine manufacturers that do that, i used to be heavily involved in competition car audio, and ran my entire system on two amps, one rated at 2 channel 20 watts/channel, and the subs on a 2 channel 30 watts/channel amp. In reality the 60 watt amp was pushing out close to 500 watts. in this case they went the opposite of HP numbers here, but it was done for a specific reason. (at the time competition classes were set by wattage, hence I competed in the <150 watt class.

the problem with standard ratings for motors is there are quite a few variables. assuming they rate for the recommended props that they themselves recommend for use, what nitro/fuel to use? what altitude? what brand of props? and assuming the manufacturer uses the average number, what do you do with the 50% of customers angry that their new engine won't get what the manufacturer says? and if you use a low number the manufacturer just shots themselves in the foot for marketing purposes.