This is the art of using your product or service's
specifications to create a misleading sense of
superiority against the competition or to create an impression of
increased value in the customer's mind.
One area notorious for this practice is consumer electronics. Unless there is an egregious difference in true performance it is very difficult to determine the relative values of stereo gear, so manufacturers often inflate the importance of unimportant or irrelevant details to create a bragging point that distinguishes their product from the competition. For example, an ad for an amplifier or receiver will tout the power output, but unless they are a respectable company they will describe the maximum power that amplifier will put out the microsecond before it melts down on the test bench. (Real power figures will always use an industry-recognized context such as average continuous output, quoted as RMS power.) That 5,000,000-Watt loudspeaker doesn't have any "Watts" at all, yet almost all speaker ads proudly and loudly claim how powerful they are. This ignores the fact that speakers are transducers that convert the amplifier's power into sound, and have no inherent power themselves (unless they are powered speakers, and then they fall into the "amplifier lies" area.) Proper speaker specs will describe average continuous power as well as the amount of deviation the sound reproduction varies from flat response in the important audible range from 20 Hertz to 20,000 Hertz.
In cars and SUVs, a company will ballyhoo horsepower and displacement, yet neglect to mention that the suspension is so lame that the vehicle would kill you if you attempted to use any reasonable portion of it doing anything more than driving in a straight line. Medicines will tell you how great they are and how much better they can alleviate your symptoms than the other product, yet blithely skim over the many (often disconcerting) side effects.
There is even specmanship in war. "Body counts" are simply boasting how much more effective our troops are at killing the enemy, and "progress reports" that state how many schools, hospitals, and miles of road were built without describing the quality and stability of the neighborhoods involved are diplomatic ways of conning people into thinking something works better than it actually does.
The best defense against specmanship is knowledge. If you understand something, it makes it much harder for people to blow smoke up your ass about it.