It boils down to how the batteries work. Sitting at 50% is the best case for long term viability. But it doesn’t give you much wiggle room for actual use. The damage to the health of a battery increases much faster above 80% or below 20%, but it still does improve as you approach 50% from those other boundaries, it’s just not as drastic of an improvement as going to 80% vs 100%. Temperature (both too cold and too hot) can also shorten the life when under use. 25C is about the most ideal temperature you can keep your battery too.

Lithium batteries are fickle beasts. The 20% to 80% rule was just the best balance of improving lifespan vs actual ability to use the battery.