It's not unheard of...in fact it's a well known fact...that the Chinese cell manufacturers flat-out lie about the capacity of their cells. Someone that takes those cells and packages them into a integrated battery SHOULD test the capacity of the cells to confirm the total capacity of the battery. Whether they do or not is anyone's guess.
My advice would be to just be on your toes about a few things when you buy a battery. If they list a mah value without stating the voltage at which the rating applies, that's a red flag. At minimum, as Richard says above, you should probably assume that the rating is either 1) the lowest voltage the battery supplies, or 2) the voltage of each individual cell 'under the hood'.
Ideally, they'd provide either a watt-hr rating or both the mah and the corresponding voltage at that mah (which you can then use to calculate watt-hr).
Also, remember that a battery manufacturer can't work miracles. This is physics and you can get only a fixed amount of power from a fixed amount of any specific chemical compound. So, if the battery is one of the 'typical' chemistries that we all know about...Ni-MH, Li-Ion, or Li-Poly and it's claiming a 2x, 3x, or 4x capacity from a package that's not also physically bigger, be wary.