[duped]

Some people put a 17.5kHz low pass and high pass at 20Hz on the master. It's dumb and you have very few reasons to do it, but people still do it and keep it as default settings in "mastering chains" that get passed around and dropped on random tracks.

So you can't be sure if you're looking at a reencoding or a lossy file or not.

[munificent]

There are definitely good reasons to high pass at something low like 20 Hz. Very low frequency signals eat up a lot of headroom, make speakers work harder, and make it more likely to encounter distortion during parts of the signal path, all for zero audible benefit.

Having what is practically a DC offset in your signal doesn't do anyone any good.

[duped]

There's definitely good reasons to do it, they're just kind of rare to see in a modern digital stack. DC offset is hard to introduce unintentionally and subsonic content is usually removed early in the mix, not on the master (if they aren't, it's arguable a bad mix).

[alternatetwo]

You can still see the difference in a high detail spectrogram like RX9 produces. There are some artifacts beyond just cutting off above 20khz.

There are also full range mp3s, since iTunes by default doesn't apply a hard 20khz lowpass like LAME does.