So why would one want a different ground plane for analog and digital?
Ummmm a bit of a long story, but the analog world doesn't like very much the switching noise generated by the digital world.
On mix-signal chips or in this case an AVR on its ADC, you end up loosing the least significant bits due to noise. How much? It very much depends on the application, but if there is a lot of switching activity or switching big loads it will be fairly big. I've measured on some stock Arduinos that you loose the 3 LSB on 10bit ADC reading due to noise. You are loosing about 19 dBFS of the 62 dBFS of its dynamic range, i.e. your noise floor would be at about -44 dBFS. All this translates into: if you used a 7 bit ADC you would get the same result. Not bad at all! So if your ADC has a 5V reference, signals that change less than 40mV would be blurred with noise. Is that bad? Well, it depends on the sensor, amplifier ... and your application.
Having a separate analog and digital ground plan joined at a star point helps significantly by having both these worlds separated (the quiet and the loud).