And I might add, the 5D3 (and I suspect the 1DX) will interpolate between wide and tele meaning if your wide is +2 and your tele is +4, midway through the focal range it will do +3.
Correct - a simple integer linear regression between the two values across the focal range of the lens. For the plot below, each lens was set to W = -10 and T = +10.
Is this a graph out of FoCal?