I’d often noticed that measuring analog inputs on my ATmega, that the first round of values usually differed substantially from subsequent values. The datasheet even warns of a similiar-sounding issue:

The first ADC conversion result after switching reference voltage source may be inaccurate, and the user is advised to discard this result.

Knowing that the Arduino libraries don’t actually set the reference voltage until the time the first sample is made, it seemed logical that these first samples were an artifact of the voltage changing. However, it wasn’t just the first sample, or the first couple samples. It could take dozens of samples before the value returned had stabilized (i.e. stopped increasing by more than 2 LSB). Being a boot-only issue. I was content to ignore it until I started trying to read the on-chip temperature sensor.