Thanks for the explanation. Makes sense, just wasn't so obvious for a newbie like me 
Regarding pinMode(), that's curious. You mean it has no effect on A6/A7 because of their analog, input-only nature?
Because the page http://arduino.cc/en/Tutorial/AnalogInputPins actually uses pinMode() on an analog pin. Ok, that's obviously to set it as OUTPUT. INPUT should be redundant, since they are input by default. But I thought it was a good measure to set them to INPUT, just in case some code previously had set them in OUTPUT mode...
cheers,
Aldo