For what I've seen the regulated voltage coming from the Arduino pass through a diode, that's the voltage drop in my case.
"Power Pins There are a variety of power and power-related nets broken out:
RAW is the unregulated voltage input for the Pro Micro. If the board is powered via USB, the voltage at this pin will be about 4.8V (USB's 5V minus a schottkey diode drop). On the other hand, if the board is powered externally, through this pin, the applied voltage can be up to 12V. VCC is the voltage supplied to the on-board ATmega32U4. This voltage will depend on whether you're using a 3.3V/8MHz Pro Micro or a 5V/16MHz version, it'll be either 3.3V or 5V respectively. This voltage is regulated by the voltage applied to the RAW pin. If the board is powered through the 'RAW' pin (or USB), this pin can be used as an output to supply other devices. RST can be used to restart the Pro Micro. This pin is pulled high by a 10k&Ohm; resistor on the board, and is active-low, so it must be connected to ground to initiate a reset. The Pro Micro will remain "off" until the reset line is pulled back to high. GND, of course, is the common, ground voltage (0V reference) for the system."
