density recompresses jpg

[dargaud]

Hello all,
From the man page:

The density option is an attribute and does not alter the underlying raster image

... except that it doesn't work for me. The command

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mogrify -density 300 MultipleSummits.jpg

reduces the file size by 10% and drops the quality accordingly. From looking at the pixel difference, I'm pretty sure there's a jpg recompression.
The same is true if I try to add a profile to an existing image:

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mogrify -profile AdobeRGB1998.icc MultipleSummits.jpg

alters the pixel values.
It's so elementary that I must be doing something wrong. So, how do I change density/profiles/... without touching the pixels ?