lithium ion - Batteries have a constant charging speed from 0%→70% instead of being any faster at 0%?

A mobile phone I tested keeps charging at the same constant speed from 0% to 70%, which is 10 watts. It uses MediaTek PumpExpress (input power: 12V 0.83A, despite the wall charger is capable of 12V 1.5A).

Why does it not charge at e.g. 15 watts when at 0%?

Another phone charges at 9V 1.67A (15W) using Qualcomm fast charging. It maintains the exact same charging speed from 0% to 60%. I wonder, why it does not charge any faster while being at indicated 0% than while being at 60%.

Side note: I actually keep my batteries in the healthy percentage range.