No, you're doing something wrong. Total unadjusted error should be within 5 or 6 counts and you are seeing 10x that.
Make sure you are respecting the test conditions, in particular in this case the internal calibration might be suspect, especially if you didn't do it. If you're multiplexing inputs a lot of other things can go wrong.
- ADC DC accuracy values are measured after internal calibration.
- ADC accuracy vs. negative Injection Current: Injecting negative current on any analog input pins must be avoided as this
significantly reduces the accuracy of the conversion being performed on another analog input. It is recommended to add a
Schottky diode (pin to ground) to analog pins which may potentially inject negative current.
Any positive injection current within the limits specified for IINJ(PIN) and ΣIINJ(PIN) in Section 6.3.14 does not affect the ADC
More on the internal calibration in this presentation.
More on ADC accuracy in this document.