Q: Why the battery capacity is 20000mAh, but the rated capacity is 12000mAh?

A: Because the input voltage of devices such as mobile phones is 5V. Inside the power bank, the original 3.7V will be boosted to 5V before output.
So the output capacity is 14800mAh ( 3.7V*20000mAh/5V=14800mAh).
There will be energy loss in the step-up and step-down process, and the energy conversion rate: more than 75%.
Finally, the rated capacity is about 5800mAh. (14800mAh*75%=11100mAh).