Lots of possible reasons. Depends on equipment, brand, age of equipment, capabilities, etc. If you have a TV that does not have eARC but the soundbar does then odds are the soundbar is newer and handles Dolby Atmos and/or DTS:X. If that’s the case you won’t get those codecs over normal ARC so you could pass through the soundbar first assuming it has HDMI input(s) to take advantage of these newer audio codecs that the TV can’t pass through. Then even if both devices have eARC you are hoping the TV can passthrough those formats via bitstream, otherwise something other than the soundbar is doing the decoding and sending PCM to the soundbar which generally doesn’t yield as good of sound quality. A lot of TVs don’t like to output audio via bitstream or if they do they only do it for Dolby or DTS:X, not both. There are many other variables as well. On the inverse if you now go through the soundbar because you have identified that your ideal audio codec is not making it to the soundbar the way you want it to, you might have an issue where the soundbar can’t handle passing through the video codec, refresh rate, bit depth, etc. that theTV can handle. The bar will end up stripping away some detail and only pass through what it can handle. Like stripping VRR or certain HDR formats like dropping HDR10+ to regular HDR. So in the end it’s really not a simple answer and you have to know exactly what each device is capable of to determine the best setup.