I though my radio ground was a...ground. Turned out to be the radio dash light 12v+ circuit (but was a hard ground when I checked it with the car off) A/C cutout?
OBTW....Your radio light is likely in parallel with the instruments lights, and with everything off, the low 'off' resistance of a bunch of instruments lights to ground will be pretty low.
And to the original post, the ignition circuit load does not typically change all that much with RPM's, and it does not know anything about throttle opening.