strong skimming increases the evaporation from the aquarium, and that allows one to add more limewater. Also, and this is very important, since the water is in better contact with the atmospheric reserve of CO2, one is less likely to get into the
regime where calcium and alkalinity are being lost from the system (which can occur when the pH of the aquarium is very high, the most dangerous time is toward the end of the photoperiod.) For the aquariums running strong skimmers, the alkalinity will be improved. Good gas exchange is crucial for limewater to be effectively used as a source of calcium and alkalinity replenishment. By promoting excellent gas exchange and increased evaporation the skimmer promotes enhanced stability in soluble calcium and alkalinity. Nitrate and inorganic phosphate are often derived from organic precursors (nitrogen in uneaten food and organic phosphates) and based on this the skimmer is probably doing a better job of intercepting organics before they are mineralized. Skimmers are pretty damned good mechanical filters, they take a lot of particulates out of the water. Those particulates may be bacteria, phytoplankton, or organic debris. All of them are going to contain some phosphate and organic nitrogen. Additionally, because the amount of limewater that may be added is increased, one may get more inorganic precipitation of phosphate, and perhaps removal of this from the system in particulate form.