I didn’t think FSD became an option until after AP2 was announced in October 2016, so I think you have a typo in your dates. I assume your S85 is an AP2 car not an AP1 car.
$3k was the original price for the package if bought with the car, so I am guessing you must have added it on after purchase if you paid more, which is very unfortunate. Tesla didn’t do their early adopters any favors with the price fluctuations.
Only thing I am glad is that I did at least order the option with my car, so there has been only one “sale” so far where the price was lower (the March $2000 fire sale). I was pretty happy with the announcement that we would get into the early beta program as a consolation prize, so them going back on that promise has been super disappointing.