They might have accidentally started with a head start on the pollution they were trying to measure.
The researchers wanted to see how much new organic nitrogen (sON) the biofilm produced during the experiment. To do this right, you’d want to start from zero. But the biofilm carriers they used were scooped right out of a real treatment plant. Right at the start of the experiment, they detected a bunch of sON and organic carbon already in the water, which had apparently leaked off the carriers.
So, when they calculated how much sON was “produced,” that number probably includes both:
The new sON made during the experiment (what they wanted to measure).
The old sON that was just already on the carriers and leached out.
It’s like trying to measure how much water you’re adding to a glass, but you started with the glass already half-full. Your final measurement isn’t going to be totally accurate.