80% protects the top, 83.6 kWh protects the bottom
The e-tron has a 95 kWh battery. But only 83.6 kWh (88% of 95 kWh) is available to the driver. I believe (but I'm not sure, if anyone has evidence to the contrary please provide it) that the remaining 11.4 kWh protect the bottom of the battery, so it never goes fully empty. So the 80% recommendation is to protect the top (so that the battery does not get fully charged). Lithium-ion batteries do NOT like being fully charged, which is why your iPhone battery loses about 15% of its capacity every year: because you use it all the time and you charge it full at least once a day. I set my charging target to 80% except when going on longer trips (more than 70 miles away), or when encountering a free charger (yes, there are still some available); in those cases I charge to 100%.