I would assume the bias pots on the DR board is for balancing purposes. A fine adjustment to ensure the A is matching the B. My Marshall Silver Jubilee and the Fender EVH 5150iii EL34 both have dual bias pots used to balance the A and B sides of the Class A/B. There is more of an issue with tubes as they are not exact matches even when matched. Also over time that tube may drift so having A and B pots makes sense. Best to use a bias probe on both sides to for ease of sanity. I need to get one of those. Also would be a good diagnostic tool for my fixed bias Mesa amps, especially when using non-mesa certified tubes.
Exact math and theory are out the window since the peak voltage of the half wave bridge circuit would be at 84Vpk not 60. However, RMS or DC voltage on a meter will not see the peak voltages as they are based on an average which will be much lower in magnitude. I would not worry about it. Once it all gets filtered and such, it should be a stable DC voltage (may have some ripple).
