Sometimes sellers have "problem" with calculating mAhs:
When they have 3 batteries with 600mAh and 4.2V connecting them together they get one 12.6V 1800mAh battery. Sadly most people get either 4.2V 1800 mAh OR 12.6V 600mAh battery.
In fact it is said even "reputable" manufacturers have this problem: capacity of power banks is often quoted as capacity of the included battery, i.e. 10 000mAh power bank in fact includes 10 000mAh battery so it is not able to provide 10 000 mAh @ 5 V.