My assumption was just that it was something about how the EQ is mixed for that jack, because it is labelled specifically as headphones. The other replies about

My assumption was just that it was something about how the EQ is mixed for that jack, because it is labelled specifically as headphones. The other replies about surround sound are true, as well, but I don't think should apply to my tv -> stereo soundbar set-up. But the difference is definitely noticeably, even to my relatively forgiving ear.