The degradation curve of lithium batteries starts somewhat steep, but then it plateaus after what I would refer to as the break in period. This is a known characteristic and it’s perfectly normal.
Besides that, this could be related to the cells becoming more out of balance from spending most of their life below 80%. By not charging the battery to full, this prevents the pack from ‘top balancing,’ which only occurs somewhere between 90-100%. This effectively reduces the packs capacity until it can spend the time to properly balance the cells. Unlike other manufacturers, Tesla does not use a ‘top buffer’ on their battery management system, which basically means the battery will spend a lot more time getting to 100% because it is top balancing the cells. For this reason, the batteries full capacity can be impacted by rarely allowing the charge to reach 100%.
Having said that, I would rather have a pack spend most of its time slightly out of balance, rather than spending excessive amounts of time at a high state of charge just to get the highest number possible.