That does make sense,  cmyk should be one of the  the last things most ppl do to an image. . . understanding what 'gamut" is and how your images e spectrumn of colours can fit  into each gamut  is the  tricky part... whatever design is done  it's often unfamiliar to make an image in cmyk mode, and often the interaction of photoshp blending modes, lighten, multiply and filters will not quite work the same... the  jpg compression would be the main factor,  --- if they use a program that is up-to-date to handle that cmyk,jpg  .. after it's opened what are they going to do with it...