Many of the batteries you can find on ebay etc are fake. The mAH is just a sticker and easily printed
A fairly good/simple way to test is to weigh the batteries. If the new battery is lighter, it almost definitely is fake.
Another way is to peel off the sticker. Usually the manufacturer will print the actual capacity on the aluminium thing under the sticker (I peeled off the sticker of a 2.99 battery that was '1500' mAH - said it was 1000 underneath).