Does the Arduino pro-micro 3.3v use an 8MHz crystal or a 16MHz crystal with a clock divisor of 2?
On the schematic of the Sparkfun Pro Micro 3.3V/8 Mhz (here) a resonator of 16 MHz is drawn. But also is stated there: "Board is marked with combination of resonator frequency and regulator voltage". From this you can deduct that the 3.3V/8MHz and the 5V/16MHz versions don't have the same resonator and that the 3.3V/8MHz version most probably has a 8MHz resonator.