AVCC is connected to VCC and both are decoupled from ground. AREF is decoupled from ground.
With REFS=01, the reference is AVCC and an external capacitor is expected at AREF. This configuration does not allow the ADC to read a resistive divider coming from a battery supply (only values close to zero are returned), but a potentiometer is read properly.
With REFS=00, the reference is AREF and the internal VREF is turned off. This configuration allows the ADC to read a resistive divider.
What is the difference? Why does REFS=01 not work?