Amazon MP3 Quality

There's been an issue that has been bugging me for years now but no one else seems to have noticed. Every time I download an album from Amazon, I'm instantly disappointed by the mediocre sound quality. I know that they are 256kbps and should sound better but something about the way they're encoded makes them sound really flat and uninteresting to listen to. The sound difference become really apparent when I recently got a Zune Pass subscription and replaced the Amazon MP3s with Zune versions and was amazed by the difference in sound quality. What makes things worst is that I seem to be only person who notices this because I can't find anyone else on the internet that says anything about this.

So just out of curiosity, am I the only one that notices this or is this a real issue?