Different LED forward voltages depending on the order of the limiting resistor on Arduino
Ok, I've found my mistake. I thought the analog input range on the Arduino 101 was up to 5V, but It's regulated to the operating voltage of the board, which is 3.3V in the 101 instead of the 5V of the Arduino UNO.

Now, if I calculate using V1*3300/1024 the readings are right, according to the theory.

Thank you all!