I got my AttoPilot sensor from jDrones, if that makes any difference. I didn't see any place for a resistor on the sensor board, nor do I recall anything from the datasheet.
I wasn't able to change the values from the Mission Planner, so…"
"I just had a similar problem where the voltage measured by the APM/AttoPilot was a bit lower than the measured voltage (~10.5V vs. 12.2V). I modified the VOLT_DIV_RATIO constant in the header until the voltages matched.