The basic clock speed is set by hardware, i.e. the crystal or resonator in the circuit, so changing supply voltage does not change that*. Basically, nothing is happening, it's just running as usual. Technically it's being overclocked, since the minimum voltage for 16MHz operation is about 3.8V per the datasheet. But, as many have observed, the MCU will likely run fine this way under normal conditions (room temperature, etc.). Well-designed and well-engineered parts will have a safety margin which will typically allow operation outside of the stated specifications. No other consequences, but also no guarantee of proper operation, so if it doesn't run correctly (instructions could be incorrectly executed, etc.) then complaining to Atmel will likely not help
*Neither does changing the board in the Arduino IDE.