I know this is stupid, but on my guitar tuner you can adjust the Mhz higher or lower. Doing so changes what note it says is sounding.

Can anyone shed some light? What is the optimum Mhz?

Not really sure, but i've always been told to set it at 440Hz (Could just be the tuners i've had though.)
im not sure either but i thought that it was the sensitivity of the tuners mic. i tune mine to 445hz and its in perfect tune like any other guitar...
I believe it's the velocity of the sound, so the lower the Hz is, the lower sounding the note would be, and the higher the Hz is, the higher sounding the note would be.

440Hz is the default though.