Just a thought on this is, that much like a plumbing system, you never get full pressure and volume out the far end, as what is available at the source. We are buying these as "juice reducers", and they do function as such, but I'm not sure we can expect them to pass the same juice out as they are being supplied and still have the current run through the maze of electronics that make them work.
I do not have the style with the digital readout but do have the style that looks just like it but without the readout. It has a three-way toggle switch that allows for direct power, regulated power and power off. It seems to show full power when the switch is in the full power position but does show a little bit less when the switch runs the current through the rheostat windings, even at full crank on the dial. None of this was with any load other than my vom.