Well there are two things there. On day 1 of the car's life, the rated miles are going to be optimistic relative to how I, or anyone buying a P85D, drive it.
I believe its stated range takes into account battery degradation but not lead feet. Correct me if I'm wrong there.
Here's an example more to your point. According to google maps I semi-recently drove about 70 miles and according to my EVSE after the trip it took 41KWh which works out to 561Wh/mile. That doesn't tell you anything about degradation, just that I have a lead foot.
Another example that might show degradation is that when I bought the car, the battery was very low so to get home, I drove it very conservatively, drive mode on chill, range mode on, and keeping it under 65. When I got home the range was lower by very close to exactly the number of miles that I had driven. My memory is a bit foggy but I want to say I got it home with like 5% left on the battery and that I then charged it to 80%. That charging session consumed 55KWh. If we assume that actual energy and battery percentages are linear then that means 55KWh represents 75% of the battery which means the battery's capacity is just 75KWh now. That's really worst case scenario because I don't trust my memory of getting home with 5% that much, I don't think the battery percentage is linear with actual energy, and I know that the EVSE number includes electrical losses and auxiliary cooling that doesn't go into the battery.