What is the lowest voltage that a microcontroller can successfully read on the analog pin?
See data sheet section 28.8 ADC Characteristics: typical offset error 2 LSB and typical gain error 2 LSB. Absolute accuracy (INL, DNL, gain error, offset error, etc) could be typically 4.5 LSB under some operating conditions. These are Typical, not Min/Max guaranteed specs. So yes it’s pretty much normal behavior. You can buy an external ADC that has guaranteed min/max performance specs. But the internal ADC trades convenience for performance. Oh and by the way, your multimeter probably has its own measurement error specs, check the booklet it came with.