eBay 3870mAH batteries that have 5080mAH printed on them?

Bought an eBay 3870mAH extended battery stock size battery and when I received it, it had 5080mAH printed on outside. I didn't know you could even buy a stock size battery with that capacity. I would think it would require a different back plate for that capacity. Is there a way to check true capacity of a battery? I did notice a little longer on time more expected of a 3870mAH battery but not what I expected of a 5080mAH battery.

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