There are more than one kind of activated carbon filters. Depending on the pore size, or any impregnation (substances like potassium permanganate, etc., check a manufacturers list to get a better ideea), the filter will retain certain impurities from that water, and release others (unfortunetlly). I'm no expert on this, but tiny break-aways from the carbon filter could explain an elevated organic signature and increased conductivity from salts (either impregnated during the pruction phase, or retained during the filtration phase). Backwashing could, probably, make the carbon retain impurities from the washing water, relesing them into the filtered water.
Just a thought, you should ask a manufacturer for details.