There are two basic ways to find a riflescope's percentage of light transmission. The first is to calculate it, using the number of lens surfaces, using knowledge of the glass and coatings, plus any other glass surfaces such as those for etched reticles, and come up with an approximate number. The other is to actually measure it using a very expensive piece of equipment called an integrating sphere, useful for a variety of optical tests. There are problems with each of these approaches, all applicable to the question you asked.
First, we don't know which method a company uses, and in fact its advertising may just quote the light transmission from ONE lens, and scopes have far more than one lens.
Second, even measuring light transmission with an integrating sphere doesn't tell us how well we'll see an object in dim light, because of all the factors I listed in my article, including our own eyes.
All of which means the numbers thrown around by various scope companies are relatively meaningless to their prospective customers, except in convincing some prospective customers that certain scopes are brighter, so they'll buy them.
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