Good example here of how you cannot use "10000mAh" as your yardstick.
You cannot use Ampere neither.
Observe the lables, the 10000mAh is 38.5Wh, the 13500 is 48.2Wh
Divide Wh by Ah and you get V(Volts) so the Volt of the 10000mAh is 3.85V but the Volt of the 13400 is 3.6V, so an "A" in one is 7% more energetic "A" than the other. Hence why mAh is a nonsense number, or only approximates to actual energy, as the Voltage of the cells is not wildly different.
Inside the phone therefore you have the affect that the phone is stating Amps which is nonsense number also itself because you don't know necessarily the Volt the phone is stepping the Watts. So an Amp is not an Amp is not an Amp.
The phone also, particularly if using QuickCharge has to work within a thermal throttle, the use of the CPU, GPU (by pressing the screen and looking at Ampere app) causes heat and the phone then responds by recharging itself slower to not heat up the battery any more than is safe. So the act of looking modifies what you see, invalidating any conclusions.
Hence Amp is not Amp is not Amp is not Ampere.
So then, how do you know anything? Well if you have other effects, like the cells deteriorate over time (Anker and your phone) and there is a roughly 5% variation by cell in manufacturing process, and you get loses which are specific to the efficiency of the phone and the socket of the phone. So the electronics are having to recalibrate. Every now and then the phone will get fully discharged, then the phone counts the Ah coming in and then estimates its % battery relative to full. This is why sometimes a phone which will turn itself off (as its calibration is off) when a cell has a lower voltage than a given % is meant to representative.