I think he was saying his trainer reads 25w low relative to his power meter, not that he's losing power when riding indoors. I connect my quarq to zwift instead of my trainer for the same reason, though in my case I believe my trainer reads high.
Sprints yesterday. Up to 1197w peak / 1067 10sec. Was gonna ride outdoors today but alas, it is raining and a bit on the cooler side.