Well i think you have part of the equation right big ben. The fan will suck the most current when it is starting or when the motor is near it's limit of power. So when you underpower them, you are actually using more current. I think you should read the article of brian256
link for the article
The part that you will want to read if the last paragraph of the voltage regulator section
Oh and by the way, if the motor doesn't turn, more current pass trought it.