Why is it weird that Dr. McCoy may have been speaking in Spock's voice? People have done other voices before -- people like Mel Blanc back in the day and Seth MacFarlane today can speak in hundreds of different voices. Why not McCoy doing a dead-on Spock?
Mel Blanc and Seth MacFarlane are doing other voices intentionally. In ST3, the implication was that it was unconscious on McCoy's part - it was spontaneous as a result of having Spock's mojo in him.