The 'output' of a skimmer, besides heavily depending on what there is to work with in the first place, can be defined in several ways though. There are several classes of proteins and hydrophobic substances, some good, some bad. Some skimmers can remove all of some, but none of another type. Some can remove a little bit of everything maybe, or alot of something else. Others may rely on extended exposure in the mixing chamber or in the main body to buffer the ORP, pH, or even oxygenation of the water, changing its chemistry. There are ways to make shorter skimmers perform like taller ones, and the time that a bubble spends exposed and interfaced to a protein can be compared at several different points... the time while bouyant in the water in the main body, or perhaps the time it is stacked in the neck and spends time draining as it rises in the foam head and into the collection cup. Many mfg's dont even know what may make their design work better than another, and in many ways they cant since they dont know how the end user will set up and adjust that product, or what it will be expected to do. The customer could decide to skim wet or dry, or to vary the input (if recirc/external) from 100gph to 1000gph, and all of these things will change the results. Its not a very exact science by any means. I think the rating system is flawed for sure though... and rating a skimmer based on tank volume isnt the best way, as a 500g with two fish in it wont come close to the waste output of a 100g with a dozen fish. Some skimmers with lower air throughputs can destroy the output of some with higher air throughputs because their design might be so much better otherwise, with cone shapes, bubble plates, extra height, etc. So there is not one exact method which gives the best results.
I was at the Shedd behind the scenes tour this saturday, and their RK2 skimmers are making them very unhappy. They are huge skimmers (a few feet in diameter, over 10' tall) with venturis and ozone going in, and their skimmate production sucks. I looked at the dwyer meter on the air intake, and it read only 100scfh!!! the skimmers were mostly just swirling some bubbles in alot of water. They had some other ones (smaller models, maybe 18" diameter and about 8' tall), that they just removed all together and decided to just do massive water changes instead. I felt bad, but I had to tell them that my next skimmer will only be 36-42" tall, use only about 85 watts, and make about 130-140 scfh of bubbles and skim the crap out of one of those RK2's most likely. I was like 'maybe you would like to try it?' or something because for all the things that those skimmers had going for them as far as the specs were concerned, their performance blew.