Looking at the picture in the FCOM it could just be that both FADEC channels can use either igniters if required but channel A sticks to ignitor A and channel B sticks to ignitor B.
Channel A - Igniter A
Channel B - Igniter B
Channel A - Igniter A
Channel B - Igniter B ....... and so on
But both channels have access to both ignitors if required. This would cause equal use of both channels and igniters?