It's not just you. I've concluded that some of the estimates must be extremely crude. On my roughly three-week-old Model X (of which I've been home only a week and a half), the battery estimate on the dashboard drops by about six miles within the first half mile of driving. Obviously, it doesn't continue to fall at that rate, or I'd barely be able to make it work and back on a full charge.
And the charging time estimates (at least at my house) aren't much better. Yesterday, I had to get something out of my car while it was charging, and I watched on the dashboard as the charge time estimate crept up from 22 hours to 23, then 24 just because of the power drain from having the door open with the overhead lights on. The car had been charging for about 15 minutes at the time, and my mental math said it should have taken about five hours to charge, which it did. I have no idea where it got a number in the twenty-odd-hour range. And sometimes when says that it is charging at 12 amps, it says it is adding 2 miles per hour, and sometimes it says it is adding 3 miles per hour. I'm certainly not an expert in modern battery chemistry, but how can the number of miles you get from a fixed charge current possibly vary by even low double-digit percent, much less 50%?
Basically, my crude in-my-head guesses, even as a brand-new, first-time EV owner, seem to be better than the car's estimates every time. I'm not sure what the heck kind of new-age math the car is doing, but I'm starting to wonder if it involves machine learning and a number line....
But in all seriousness, I think that when you check the voltage of a battery right after charging, it lies to you a bit, because the charge hasn't fully distributed through the entire cell/pack, so you get an artificially high value upon initial measurement that quickly dissipates as soon as you put the pack under actual load. This also means that when you tell your car to stop charging at 265 miles, you're probably stopping several miles short of that, and the shortfall likely depends on charge speed, so superchargers would give you the most loss right off the bat. Or at least that's my best guess about what's happening here.