To me, it just sounds like some of the instruments were erased from the original recording leaving a stripped down "acoustic" version. I've compared Bono's singing and I think the vocals are exactly the same. But from what I can tell, and I'm no music expert by any stretch of the imagination, Adam's bass guitar and perhaps some of Larry's hi-hat seem to have just been erased leaving what was left from the original album track. I'd like to hear other opinions on this too, and don't take mine to be absolutely correct, but mine is of the latter of your question... an "alternative mix" of the album track (even though an "alternative mix" implies more work done on the track than I think was done).
