i bet its worn out. especially for just a single stage, it will be worn out after a couple of use.
my previous unit also throw out higher ppm as compared to tap water. this, i believe, could due to the resin is at its max capacity and tend to throw back binded ions back into water when a more "attractive to resin" ion is in the columb. i could be wrong, what i mentioned was just based on my chemistry knowledge till date.
when this happens, change the cartridge or change the resin.
happy reefing