IMO, because there are specs for batteries and there are testing methods. None of which are very well "policed". You can produce a product and claim pretty much any performance specs you like and will have no accountability unless someone takes you to court over it. Meanwhile, you will be making money hand over fist before you eventually lose the court battle because you had misleading advertisements.I keep asking myself the same question.
There have been tons of independent test labs that have tested batteries. They're all over the web. Google.... Anyway, the bottom line is that any battery rated within about 20-40 percent of another stands a good chance of having approximately the same tested specs. To "break out" of the "specification game" uncertainty, you pretty much gotta double or half the specs to be assured of getting a product significantly different than the one you are comparing it to.
-Frank