i have a PolyTune 2 and I'm wondering what is the accuracy when in poly mode vs single string.
The reason why there can - at times - be a slight mismatch between the polyphonic and chromatic mode on PolyTune is that we use two different ways of measuring the pitch. In short, the traditional way of detecting pitch in a regular monophonic tuner, will show the pitch rise quite drastically once the strings are hit. As they ring out the pitch will slowly drop in pitch.
The polyphonic mode on the other hand measures the strings overtones, which will not go sharp in the same way. This means that you see the "true" pitch of them right away which is lower than what you initially see on the chromatic mode.