Most cars have the ability to re-calculate based on actual usage. So You're saying that Apple cannot master this seemingly simple algorithm?
Well, it's not that simple.
While cars are one of the closest analogies, like all analogies, it's not perfect. I think the variables are different, and for the most part observable and understandable on vehicles. If a range says I have 200 miles left on a tank of gas, but I sit in bad traffic for an hour with the AC blasting, I'm going to get a hit on my gas rating but I'm going to know I got stuck in bad accident on the freeway.
If you've ever had a car that has this range, or even gives your current MPG, you know you can play games with the readings. Whoo! I'm going downhill, I'm getting infinity MPH!" Or, I Just slammed on the gas and I'm getting 10MPG."
Now, there is something with Sierra on battery life. Read through my post history and you'll see me complaining about it. But even on El Cap, I've never truly trusted the "time remaining" gauge. In both directions. I've had it last less than the time showed, more than the time shown, and I've had it charge in well under the estimated time. Even on El Cap, I spent more time analyzing the time remaining than I should, when I don't really care it's not on my iPad.
There are more apps and processes running on my laptop than on my car. I know, I know, cars are all computers but I can't download an app onto my car's fuel injection system, and it doesn't get updates (unless I bring it in for service).
The battery life gauge is like the MPG reading when you're coasting downhill or slam on the gas. For giggles, on El Cap I pulled the plug on my Air while I'm typing this (it's idle. I'm typing on this on another computer). In the 3 minutes I've been working on this, I've gone from 7:31 remaining to 7:48. And that's idle with Safari running in the background. Refocusing back to Safari brought me to 7:38 remaining. I just opened up Word and went to 7:28. Kicking off an Office 2016 update I'm now showing 6:38 remaining.
The only measurement that's mattered is "I did these typical tasks and my battery lasted x hours." I've always seen how long it drained down to 75% (assuming I'm not playing a graphics game) and based my battery life on that.