Mine battery seems to be following the same trend. Also a 9/19 build.I have a LR AWD built in 9/19, and have had seen my rated miles drop pretty steadily since getting the car, and am currently sitting around 295 miles of rated range at 100%, with 4000 miles on the odometer. To try and better understand things, I started tracking nominal full pack with Scan My Tesla at the beginning of December. I had 74 kWh at the beginning of December with 3000 miles on the odometer. Today, with 4000 miles on the odometer, I have 72.8 kWh. The decline in my rated miles at 100% and nominal full pack values correlate very strongly when graphed out vs. time or mileage, and it seems likely that the two are calculated in a similar manner. Extrapolating the trendline for the nominal full pack curve backwards to when the car was purchased gives a nominal full pack of 76 kWh.
I live in the San Francisco Bay Area, and the car the garaged at night, so my max temp swings are about 50 to 80 degrees at most, so temps really aren't much of a factor. I'm willing to see how things fare over time, but I'm pretty sure my battery is going to be one of those that makes everyone else's batteries look good. It seems like a lot of change for such a short period of time (4 months, 4000 miles).
I have seen nominal full pack values on you tube (mostly on OBDII harness installation videos) range from 68-77 for LR Model 3's