Guess-O-Meter or Accurate Range Display?

That's why it's called that. Because "all things being equal" doesn't. So while it's literal accuracy within that constraint may be good, the circumstances so seldom apply that it's just guessing at the next trip, which is the only one people care about while looking at the number.

The thing corrects for the amount of juice left in the battery as you drive, but doesn't much alter its math for how far you can get for a given amount of juice during the drive. If it thinks you can get 4 miles per "voltage span as represented by a segment on the battery display" (hereafter "segment") and you go from 8 to 7 segments, it'll show remaining range correspondingly down as each quarter of that voltage span drops away. But it's not going to note that you only drove two miles since the last segment and halve the estimate remaining accordingly, because maybe that was a hill and now you're going to go down it. The car doesn't know, so it doesn't try, because trying would lead to VERY wild swings in the estimate, and make it even less useful for most drives and drivers. And it's up to you to figure out how that's going to be different if your drive isn't a closed loop on a single charge, and what conditions are going to be different from the last one you did.

And it doesn't even really know how much in or out, either. It knows voltage and how much is flowing at that instant. It can time-slice the flowing at that instant, rack up a couple ten thousand readings over an hour or so and have a PRETTY good idea how much came in or out, but that frequency of taking readings can always miss something. Voltage tells you where you are between full and empty and when you need to stop doing what you're doing and do something else, but again, that doesn't tell you WHEN you're going to have to change what you're doing, because the consumption and charging curves aren't perfectly flat either. There's a lot of math and estimation between the two things.