According to the xmega a datasheet, the unsigned transfer function is
RES = 4095 * VIN - (-dV) / VREF
dV = VREF * 0.05
RES = 4095 * ( VIN/VREF + 0.05 )
which means that the offset (5% of 4095) is independent of VREF. Now, I know that the 5% are not written in stone, and that's fine, but what puzzles me is, that the offset seems to depend on VREF :shock:
Here a chart that plots RES vs. VREF on a grounded input pin (hope the document is accessible to everyone).