And actually a float’s position on a vertical rod when measured is also a guess and in some cases not a very accurate one.
I don't think I would say that a gas gauge is guessing. It is an instrument taking a reading. Like any instrument the accuracy depends on the particular instrument. You wouldn't say that a speedometer is guessing how fast you are going. It is taking a reading, with several variables that can affect its accuracy.
An EV range estimator on the other hand has to at least partially guess, because it has no way to know what the conditions will be on your next trip. It starts with an instrument reading of the SOC, then it looks at data history for recent trips to calculate what was the average miles per kWh on those trips, perhaps weighted towards the most recent trip. But after that it has to essentially guess what the conditions will be on your next trip. Will it be uphill or downhill, 70 mph or 40 mph, 85 degrees Fahrenheit or ten below freezing. Even though it knows the current temperature that doesn't help because it has no idea when your next trip will start, your next trip might be in one minute or in one week. And how will the car be loaded, just you, or the whole family and the husky and all of your luggage and a 50 lb sack of dog food for the husky. You may know some or all of that information, but the car doesn't, so it guesses based on your most recent trips. Or to be more accurate it "guesses" that your next trip will be under similar conditions as your most recent trips. Gas gauges don't do any of that, they just take a reading and report it, with the accuracy depending on the instrument.
Future EV range estimators will probably get more sophisticated and factor in the current planned route including speed, topography, predicted weather along the route including wind direction (based on your input of estimated departure time). Tire pressure (which affects range), maybe even the current weight of the car. But to make it really accurate you would need to be able to input what your intentions are in regards to the speed limit, because going 15 mph over the speed limit will have a big impact on range. But I kind of think car makers are not going to have a setting in their NAV system where you can inform the car that you intend to speed, seems like their lawyers would have a say about that. I guess the system could notice what your previous habits are in regards to the speed limit (just don't tell the lawyers). But wait, maybe dad likes to speed but when mom is driving she goes the speed limit (or vice-versa). Since it doesn't know who will be driving, the EV estimator will also have to be tied into the seat memory system, i.e. who is driving today, fob #1 (law-abiding spouse) or fob #2 (speeder spouse)