I wouldn't blame internal pullups ..
Usually microcontrollers have just one A/D converter module that is shared by all ADC pins and on top of that there is the S-H circuit .. see picture below ..
So, voltage levels that are present on the other inputs - after reading them - will affect this unconnected input reading as well ..
However, in my case,
throughout my whole program i select just that single ADC input, and never read from the other inputs. Hence, there should not be any effect from other inputs!