To have a wider appeal.There's the album version wich I assume the live performance will resemble the most.
There's the sigle mix wich is a bit harder.
and there's a slower mix of the song for Adult contemporary radio.
It's all about the airplay.they have the album the and the songs on the album the way they wanted them.but when releasing singles it all comes down to airplay and sometimes the song gets changed to achieve as much of it as possible.