Hi,
If the ADC has several channels such problem could appear if there is no time to settle down the voltages.
I mean, if you measure channel 1 then 2 and so on , then you need time after you select the channel to charge the internal capacitors of ADC. Only after that time you can start the conversuion. If you dont' do this, then you read wrong values like in your case - they are somehow connected wuth input voltage, but they are not what you want.
Regards
Luben