Even without changing words - lyrics (ditto poems, novells, stories) can be interpreted differently by performers OR audience - based on the performers or receivers interpretation. This can be true even WITHOUT verbal/aural interpretation - but directly from printed word. the same is true of paintings.

But when music moves off the printed page and becomes a performance piece, a whole new level enters into the equation.

With all due respect to Harvey - because I can understand his feelings - the lyrics he wrote might have a completly different meaning for the artists performing them.

In essence - unlike a book or a painting, which remain static once finished - a song is mutable. Poems can be very different based on the delivery even when no words are changed - likewise songs.