There's other things that happen to a battery as it ages. Voltage sag with heavy current draw is one. An app that measures capacity may not take this into account. It may look fine and be at full voltage with light usage, but then launch a CPU-heavy app and it drops down a few tenths of a volt and isn't able to hold the voltage.

If it's discharged enough, sometimes this will drop the voltage enough to cause the phone to shut off even if it has a decent charge, say 20-30%.

You'll notice this in reverse too. The battery will seem to start charging faster than usual, but then the rate drops off dramatically as it gets close to full.

As a drone enthusiast, I'm pretty familiar with this effect because we abuse the shit out of our batteries.

85% after 2.5 years is not good. My car battery has guarante of 80% capacity after 6 years. 20% of range is a significant difference so I take car of my battery and don't charge it above 80% if not needed. It's the same with laptops. Current models can easily last 5-10 years but having only 50% of capacity after that time would be a problem. Sure, if you're intending to throw it out after 3 years it doesn't really matter but if you want to use it for as long as possible you definitely should take care of the battery. It's pretty much the only part that degrades (except maybe keyboard).

Stopping charge at 80% (and still going down near 0%) should give you roughly 4x the cycle life. So in theory doing that it would take about 8 years to hit the same 85% usable level.

85% left after 2.5 years is a high rate of wear, due to phones really pushing as much as they can into the battery to have longer run times.