There are two basic designs of APU. You can either bleed air from the compressor, or have a separate load compressor.
The Garret 85 in old B737 DC9 etc had a bleed from the engine. In this type of APU the APU works harder the more you bleed off it. So no packs on is the lowest fuel useage.
The ST6 on the Tristar had a load compressor. This is a separate turbine and compressor bolted onto the engine. It spins all the time and produces air. If the APU bleed valve is closed then this air is dumped overboard through a dump valve. On the Tristar the lowest fuel flow is with the APU set to min bleed, and a couple of packs on. Turn the packs off and the APU works harder, because the air is being fed into a closed duct.
So look and see what design of APU you have.