I expect the AXi powers not adding up is an issue with either the firmware or hardware within the PSU. I have posted on the Corsair forum about this several times and finally for a reply, see http://forum.corsair.com/forums/showthread.php?p=845107. There are also limitations in the PWM power controllers and any +3.3 / +5.0 / +12.0 / PCIe currents < 4.5 amps (AX1500i+AX1200i) or < 3.0 amps (AX860i+AX760i) get reported as zero.
I have written code to report all the Corsair AXi, HXi and RMi PSU information, but only when they are C-Link attached. If you get us a USB trace of CL reporting your RM850i then I suspect Fiery and I may be able to figure out how to report your PSU when it's USB attached.