SPimenta:
I connect the USB (from computer) to the Arduino and when I measure the voltage with the multimeter the Vin pin (which is the voltage directly from USB computer) ...
As Lefty said, no it isn't. The signal XUSB goes via MOSFET (T1) directly to +5V. Any voltage appearing on Vin would be parasitic. A 0.7V voltage drop would not be all that surprising.