I just discovered that the INTERNAL analog reference voltage for the Arduino Micro appears to be 2.56V instead of 1.1V (like for most of the recent Arduino boards).
On the page describing the analogReference function, there is apparently no information for the ATmega32u4 (used in Micro and Leonardo) :
INTERNAL: an built-in reference, equal to 1.1 volts on the ATmega168 or ATmega328 and 2.56 volts on the ATmega8 (not available on the Arduino Mega)
I could not find any information either on the Micro and Leonardo pages. I therefore wrongly assumed that the built-in reference should be 1.1V (like most the recent boards), which will cause me some problems with a circuit I just built...
BTW, I had to dig to page 292 of this document to find the internal analog voltage reference for the ATmega32u4. It might be a good idea to update the Arduino page describing the analog reference function for the ATmega32u4...
Thanks!