Antenna gain claims.

Over the years, many antenna have been available in the shops. They often give a gain figure, which can vary from close to zero to double figures.
But, just how reliable are these figures and can you rely on them to decide which one will perform best for you?

True or false?
There is a saying regarding "special deals" that applies equally to manufacturers claims...
  If it seems to good to be true, it probably is.
There can be no doubt that some of these figures look quite incredible, such as 18 Inch antennas claiming a high dB gain, but these are not exactly lies. They are calculated and written in ways that may lead non-technical people to think they mean more than they really do.
It can be a bit of trickery, smoke and mirrors, spin or simply being economic with the truth - it's not only what they tell you, it's what they don't tell you that makes the difference.

Sales techniques.
Before calling dB figures false or accusing a company of lying, consider whether you can prove it. Unless a company does not have the finances to obtain real figures, there is no reason to lie. With a few tricks they can manipulate the truth, make the figures look better.
I suppose it's no different to companies adding colouring to food and drink in order to make it look better. Those additives do nothing to enhance the product, just your perception of it. Marketing people know how you will judge a product, their job is to do whatever it takes to improve sales.
If the target customer judges a product purely on it's gain figure, then that has to be improved. But, there's good news and bad news for the manufacturers:
Bad news: After years of designing antennas, it cannot be easy to improve on the gain already achieved by existing product.
Good news The target customer does not really understand the figures, so anything with a higher number will sell the product.

Is it their fault if people do not understand the facts or figures, yet still try to rely on them and end up being mislead? OK, so they know exactly what they are doing with their figures. But, now, you can too!