Well, I think it might have to do with the length of time your tank has been established, and your fish and coral load. In the 2 pics I posted, the first one was of a well established reef tank with good fish load and low coral load, but the tank had been up for 3 years and the skimmer was extremely dirty and had recently been fully cleaned.
In the second tank, that one had no skimmer for 3 years (and no significant cleaning for 3 years) and I hate to tell you this, but I checked it 24 hours after I fired it up and there is already sticky froth at the top and about 1/2" of nasty brown skim in the cup. So I'm guessing you don't have a lot of DOCs.