It's much worse than this. One year can be enough for a 20% drop like that, and it's not impossible to go even lower. Think 50% or worse after 'some years' of aggressive use.
Anecdotally this feels like what happens with phone batteries. Apply tells me my battery is at 80% health, but when it's half dead by lunchtime, I'm pretty sure I'm getting more like 50-60% of the original life out of it.
It's much worse than this. One year can be enough for a 20% drop like that, and it's not impossible to go even lower. Think 50% or worse after 'some years' of aggressive use.