After doing quite a bit of research I eventually stumbled upon this YouTube video.
https://youtu.be/tC2Klazem3g
It seems to me that a cold cranking compression test (without spark plugs) does give different readings between the two banks of approx 20psi. The only plausible explanation I can find is that the cam phasers start off in different positions and only start to phase when whatever conditions are satisfied.