You're confusing frequency and resolution. Just because a set of music fits in a certain frequency range doesn't mean that all representations of it are equal. For example, consider this in graphical form:

https://i.imgur.com/ZgU1bgI.jpg https://i.imgur.com/RZpbyyI.jpg

Both have the same 24-bit color space. Both have the same blackest black and whitest white. Both have the same resolution. And yet one preserves more detail and information. This is the nature of lossy compression.