Remember infinity laws. You have a centrifugal fan. Infinity laws apply to centrifugal fans and they say that the flow is the square of the speed and the hp is the cube of the speed. HP = volts x amps. DC motor speed is linear to motor terminal voltage.

So that says that a 5% increase in voltage will increase hp requirement 16% and a 10% increase in voltage will increase hp requirement by 33%. You get the percentage increase in volts back as an amp decrease, so for the 10% increase in voltage, amps goes up approximately 21%.

This is theoretical and real world will vary some but not that much.

Bottom line is that a small increase in voltage increases amps quite a bit, so if your alternator voltage went up the fan was drawing more amps. That would explain your bench test results differential versus in the car results.