If it is any consolation, this has been bugging me for 30 years and I am still not convinced.
A idealised ADC converter would have an output which, if you plotted it on a graph as a function of output versus input, would be a straight diagonal line.
An actual adc output looks like the side view of a staircase. The theoretical diagonal line might be touching the top corners of each step, or the bottom corner of each step, or slicing through the middle or each tread and riser.
Also, the top or bottom step might be the same width as the others, or only half the width. Also, the staircase might end at the top end with a vertical step rather than a horizontal one, which might be incapable of being represented in the output available. After all, there is no really good reason why 1024 could not be a valid output for 5.0 V, it is a 16 bit number after all.
If I was to answer this question, I'd get a very stable adjustable voltage source, and actually measure the behaviour of the device. Is the bottom step a full step wide, or only half a step wide ? And so on. Once I had deduces the apparent position of the staircase, relative to the nominal diagonal line, then you are in a position to map the adc outputs back to the nominal voltage, represented by the midpoint of each step.