What Happened?

Music is recorded in stereo which means that two microphones
are used to record the music, one microphone for what will
ultimately play from the left speaker and another microphone
for what will play on the right speaker.

Stereo recording gives music a fuller sound when you listen
to it. As you stand between the two speakers and listen to the
music you may hear a violin over towards the right, and you
may hear a flute over towards the left. Generally, you'll hear
the vocals in the exact center, even though there is no speaker
in the center, it's an illusion.

When the music was recorded, the vocalist is generally
standing in the center directly between the two microphones,
and the music is generally somewhat different between the
left and right sides of the stereo.

The vocal remover software cancels out whatever is exactly
the same on the left and right sides, but leaves what is unique
on the left side and the right side. The result is that usually
the vocals are removed since they are the only thing in the
center of the stereo and the music is preserved because it
is usually unique and different between the left and right
sides.

However, some music may be recorded in a non-standard
way, sometimes concert or live recordings can offer different
results because:

1) perhaps the vocal is not centered
2) perhaps there is another instrument exactly in the center
like a live non-amplified guitar being held by the vocalist
3) perhaps the song wasn't recorded in stereo, which means
everything from the left speaker and everything from the
right speaker is exactly the same
4) perhaps an extreme electronic echo effect (reverb) was
added to the vocalists voice

In situation 1, the vocal would not be removed since it is not
centered between the left and right speakers.

In situation 2, the instrument exactly in the center would be
removed along with the vocal.

In situation 3, the entire song would be removed, resulting
in silence.

In situation 4, much of the vocal would be removed, but the
unique "spill over" from the echo would remain on the left
and right sides. (However, many customer like this effect
as it sounds like a backup singer once you sing over the music.)

Read more about these types of issues here