There's Pro-Mini for 3.3V at 8MHz but not 16MHz.
Check the Atmega328 datasheet.Atmel states that the ATmega328 cannot run at 16 MHz with only 3.3v It can't work
Then why would they say it can't run at that speed at that voltage?
They know from testing that some of their parts will fail under those conditions. They want to be able to tell people who run under those conditions that they clearly state in the datasheet that you should NOT try to run under those conditions. "You cannot expect to hold us responsible for systems failure when you run the processor outside the specified parameters."