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Note that different JPEG software will use different levels of chroma subsampling, independent of the quality setting.

XnView defaults to 4:2:0 subsampling, which means color resolution (the UV part of YUV) is halved in each dimension. You can change this to 4:4:4 in the compression options (it calls it "1x1, 1x1, 1x1" instead of the name everyone else uses, 4:4:4), which will disable subsampling. This means that XnView JPEG is lower quality, even if you raise the quality slider, unless you know to fix the subsampling.

Google says Photoshop doesn't use chroma subsampling above quality 7 (this may vary across versions). I've confirmed this at 11-12.