MP3 and AAC encoders typically don't have enough headroom to handle the very high peak level of complex and nuanced recordings , and so introduce extra clipping distortion as well as all the encoding artefacts. This is what the Mastered for Itunes initiative, now rebranded "Digital Masters", have been trying to avoid, combining the guidelines and tools provided by Apple with their state of the art, and very much improved trough the years, AAC encoder.
Again, all the technicals details can be found
here. Is not just marketing, it´s a good thing. By the way, that PDF have been just updated, and it now says this at the beginning
Note: Apple Digital Masters replaces Mastered for iTunes as the name of the program to better reflect the fact that these audio advantages are available to our entire music catalog across the Apple ecosystem, whether streamed or downloaded. All Mastered for iTunes releases are now badged as Apple Digital Masters. All of the Mastered for iTunes software tools are still usable for Apple Digital Masters creation.
