The fact that you noticed the highs having the most prominent difference is not surprising. I take it you're familiar with the digitizing process and how a CD contains 44,100 samples per second, and each sample is a 16-bit 'word'.
Now picture two soundwaves, one of an upright bass playing a low note and the other of a brushed cymbal. There's a lot more going on in the brushed symbal soundwave. In addition to being more textured, the sound is much higher in the audible spectrum and thus oscillates many more times per second than the bass note. The more complex a soundwave, the more is potentially lost upon digitization. (IMHO, CD audio is a compressed format since it discards data by design during the mastering process.)
This same phenomenon plagues MP3s. If you ever listen to MP3s, the highs are where you notice most of the compression artifacts.
