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Another theory here is that the speed control on the fan can be controlled by resistors in series with the fan motor. The resistors' job is to drop the voltage in the line to reduce the fan speed, and release the extra energy as heat into the heated air stream in the duct. Three resistors of a different resistance produce the three lower speeds and then when switched to maximum speed the fan has zero resistance in series with the fan motor and runs at it's designed speed and current (Amps), and it's maximum efficiency. This could account for your drop in current at the fans highest speed.
As a side note, sometimes a vehicle defroster fan will not work on any of the lower speeds but will work on maximum speed when the resister module fails because full speed is on a different circuit.
All of this being said, I would think that a late model vehicle would have a more sophisticated electronic speed controller. But maybe not, since that would be much more expensive than a simple resistor.
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