The photo sensor in the metering is likely human vision range and the paper is sensitive to mostly green and blue. Any out of paper sensitivity band light falling on the sensor would appear in your reading if within the sensor bandwidth and above its minimum threshold. Since the safe light 'lux' would be constant it is likely a slight offset to the actual light through the enlarger contribution. It would hide from you until you made a safe light off reading. Either light on or light off numbers work on their own. It is so much apart of life that the full environment is active in what we do. And much of it hides in the circumstances quite well.