Southpark:
Do that in the online calculator and see how many decimal places the result has.
Do the math and see how quickly the digits reach insignificance in the face of noise alone.
Marvel at the number of places that a calculator can show that have already been shown to not matter.....
The way it works is... 10 bit... 1024 levels. Level 1 to level 1024. Voltage range is 5V for the analog range. 1024 levels (steps) contains 1023 individual gaps..... just as 2 steps has 1 gap. We don't divide the 5V range into 1023 portions. Instead, we divide it by the total number of levels (ie. 1024).
The way it really works is all that and error. You forgot the error that is bigger than your point.
Oversampling is how ADC accuracy gets improved. That or get a better ADC but by 12 bits the wiring gets real important.
It's not like there is only ONE WAY to treat analog reads. If you're a stickler for accuracy then first divide by 4 and then get upset over the divide by 255 or 256 because then the difference will be more significant than the noise. Sure you lose precision but that's not accuracy.