Then there is an offset of 0.5 in all your results. That will be significant if this is a battery voltage represented in volts.
The offset is 0.5 LSB. If it is significant something is very wrong (typical(!) offset of AVR ADC is 1.5 LSB according to the speccs). Also I have feeling adding a constant cannot add any real information anyway. It just changes meaning of the numbers - you may compensate in postprocessing.