That's the thing about aerodynamics...it doesn't seem to be enough to use the "ratio of something" and just compare them if they are of different sizes because aerodynamics does not scale linearly. And depending on what the fan was used for, it may have mattered. I mean...how many people actually check the the efficiency and efficacy of their computer fan? Sure, people might say it, but how many people actually check it?
Not sure if this applies here...but I was reading on a plane forum that pulsejets does not work at small scales because the size of the engine is so small that the static boundary layer fills up a significant volume of the engine compared to a larger pulsejet. Maybe something similar is happening here with the computer fan (if anything is really happening at all). My guess is that it's just cheaper and no gives a crap enough to actually check in computer fans- everyone just checks temperature, and temperature is easy...just give the fan more power and the temperature will be lower.