Taking a fairly extreme case.
Let's take a 90 gallon display with a 40 gallon sump, sump isn't full to the brim and there's rock in the display, so call it 100 gallons actual water.
Say you have an extremely large skimmer cup (14 cups in almost a full gallon!) and it fills once a week with not much gunk and pretty much all water at 1.026.
After one week, your sg would drop from 1.026 to 1.02574, two weeks would be 1.0255, and three weeks 1.0252.
That's pulling a gallon of skimmate a week of what appears to be a relatively unrealistic sg of the skimmate.
I would venture to say that although your skimmate does no doubt contain a small portion of saltwater in it, the design of a skimmer should drain the saltwater and only skim the organic crap for the most part, and anybody running into a significant swing with their sg because of their skimmer, no matter how "wet" you skim, has a serious skimmer malfunction, or the cause is probably somewhere else.
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