Not that I really have any idea, but maybe could it have something to do with how fast the DOC accumulates into the water column (i.e. the
rate of accumulation) and varying efficiencies of a skimmer to pull it out as it happens... ?? So, say for example if skimmer A is known to outperform skimmer B and we have both skimmers running on a tank, as the fish poop into the water the DOC (or whatever it is we are pulling out with the skimmers) starts to accumulate. Skimmer A starts pulling the DOC out before skimmer B does; thus suddenly the rise in DOC starts to taper off a little bit. Skimmer B doesn't pull out "nothing" per se but it is less than that of skimmer A. Assuming no more fish poop for the moment the DOC starts to drop a little bit and thus for the moment there's nothing new to pull out. Until another fish poops again, and so on.
IF this theory was somewhat close to reality then it would suggest that two skimmers of identical abilities would both be pulling out gunk equally.
But .... I have no idea if it "really" works this way at all or not. I'm just thinking out loud for the sake of seeing if anyone else has a theory about it ...
cheers