My experience with an Apple battery saying ~81% longevity remaining is that it'll die when it still reports half full and you open a demanding webpage

It's a genuinely hard problem to measure battery capacity with existing smartphone hardware, also because it's a matter of opinion how much to factor in the peak load capacity (how do you count the bottom 40%, where it can't handle peak draw anymore? Should one include half of it because the phone is still usable but in a degraded state?), so I'm not faulting Apple here at all. They choose to display this estimate and it's better than nothing / better than most manufacturers. Just that you can't take it at face value, even if you charged your phone from 0% to 100% for >=1000 days