Why is the actual mAh of a powerbank less than what is specified?

For example, a 15,000 mAh powerbank can only fully charge an iPhone X (2716 mAh) 3 times before the powerbank needs to be recharged. My iPhone X would have only taken 8,148 mAh after its 3rd full recharge from this powerbank.

This power bank is considerably new and has only been fully charged 3 times since I got it.

Is it possible the battery inside the power bank still needs to be conditioned by discharging and recharging it a few times?

Hi so every thing comes at a cost. Facts are it is a storage device which is only as good as it's cost. Yes quality is about what such devices cost in the first place. Sounds more like your either not telling us the whole truth or you have a defective I Phone X or it just goes to show not all new technology is not that great.

Amp-hour capacity is a function of the rate of discharge, with the rated value for the optimal rate, which is very slow. Discharge it faster, you get less capacity.