The mAh rating of power banks are actually of the lithium battery inside at the 3.7V rating of the lithium-ion Li-Ion or lithium polymer LiPo battery. The rating is not of the output at 5V, which is stepped up. In addition, power banks are not 100% efficient, so some energy is always lost.

What is the real output capacity of a Power bank?.

I happen to own the IMUTO 10,000mAh power bank in the photo which shows Capacity: 10000mAh/37Wh (i.e., at 3.7V) on the back:

I have another ADATA 10,000mAH power bank which shows Battery Capacity: 10,000mAh(36Wh) (i.e., at 3.6V) on the back. This power bank also shows Rated Capacity: 6,300mAh(31.5Wh) (i.e., at 5V) that indicates an efficiency of 31.5/36 = 87.5%.

For a 20,000mAH power bank, it is reasonable to double the above.

EDIT:
Corrected efficiency calculation 31.5/36 = 87.5% instead of 31.5/26 = 87.5%.

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