All ADCs I have looked at document that they can't represent the positive full scale. For instance, for an 8 bit ±1 V ADC, -128 represents -1 V, +127 represents 127/128=0.99219 V. The transition from 126 to 127 happens at 1.5 LSB from the positive full range. 1 LSB difference represents 1/128 = 0.00781 V difference, and not 2 / 255 = 0.00784 V.
But if you actually care about what the voltage (and uncertainty) is, most of this is difference is mostly pointless, you're reference will have a bias, there are linearity errors and so on. 1 LSB will not match either the 1/128 or 2/255, you will need parameters to compensate for it.
But if you actually care about what the voltage (and uncertainty) is, most of this is difference is mostly pointless, you're reference will have a bias, there are linearity errors and so on. 1 LSB will not match either the 1/128 or 2/255, you will need parameters to compensate for it.