Yes this is serious question. Why is it not always the same when you fill it up.

Like jslazart wrote, those numbers are not very accurate and are based on many factors, based on data acquired from various sensors by the on board computer, such as highway vs city driving, engine load, temperature, how much you floor it (or not) when accelerating, how long you let it sit idling, lots of stop-n-go or not. They will tend to get more accurate after a few fill-ups, IF you are using your car mainly for your daily commute. If you are using your car for other occasional uses like going to cottage on the weekend, lend it to your son/daughter, go out of town a lot, those readings will likely always be all over the place.

Unfortunately, the best way to accurately manage your fuel utilization is, still, the old fashioned way of writing down your mileage on the receipt at each fill-up.

Some of them are not so inaccurate. I got a Honda Civic a year and a half ago and checked the reported mpg vs. calculated by me, each time I filled it up, and it was within .5 to 1 mpg over about half a dozen tanks. So I quit calculating it myself. But the instantaneous mpg and the miles-to-empty will change as you drive. Notice if you reset the trip meter, the first few miles the mpg for the trip will change radically because every mile is a large fraction of the few miles that you've driven. Same with the miles to empty. And it will change as you go up and down hills because the gas gauge is seeing a changing level in the tank.

If you haven't had this kind of a car before, you'll get used to it. I obsessed over the readouts for awhile but now I just glance at them now and then.

The car has 75k on it. Is it new to you or have you had it awhile? The miles to empty does fluctuate based on typical driving. The average MPG is a big player. I recently did a 1500 mile drive and traffic and weather cooperated so we were really efficient mileage-wise. We filled up and the miles to empty was higher than we've ever seen it on the car probably by 100 miles or so. As others have said the number doesn't start there and subtract from miles driven, it adjusts on current usage and fuel remaining too. As it gets close to empty the indicator should reflect the lesser miles available. If you've ever driven it down, I was surprised to see that on my 2016 Hyundai when I reach 30 miles to empty the indicator changes to "---".

Oh this is good information. The car is used but new to me.

The car may be using adaptive software to compute 'estimated distance to empty'. This kind of programming averages the past few fill-ups to get an average mileage and uses that number to estimate how many miles to empty. The one number it has for this computation can be suspect and that is the number of gallons consumed per mile. It has no way of knowing how much fuel was actually put in the tank. The only information it has is the signal for the fuel gauge and this is purposely not linear. Most automotive fuel gauges tend to fall faster during the last half of indicated amount. Some gauges may show near empty when there is 2 gallons left in the tank. This is done by tapering the resistance element on the float rheostat. This was done to protect against running out of fuel and to keep the fuel pump submerged for cooling. If the software assumes that the gauge needle is the actual fuel amount when it is non-linear, differences can occur especially if the tank is refilled at differing fuel levels. The 'miles to empty' is an estimate only, and not true value. For example, suppose the car has been driven easy and has a history of good mileage, and then on the next tank, it is driven at high speed over hilly terrain and with a headwind, the actual mileage will fall.