The best way to test fans would be to use an electric motor to spin the fan and measure the current required. That will tell you the power consumed without having to use a dyno. An engine dyno isn't a very good tool for measuring small power differences, it is more of a gross measurement tool. Kind of like a yardstick. There are lots and lots of things that can change from pull to pull which is why an engine dyno can have problems measuring this kind of stuff.