I would be very interested to see the way you have used to generate a frequency of 6MHz (as you said), I say it's impossible.
The max frequency you can get from a AVR pin is about 500KHz, the only exception is the SPI clock which can go up to CPU/4 but you can't have a constant square wave using it, you will have small pauses between each byte transmission.
I stand corrected, yes CTC is another way that can be used for pulse generation.
Your code should be able to give an output very close to 7MHZ with a 14MHz crystal so what is the problem?