They just play lower so Deryck can sing the song more comfortably. Many bands do that when they play live, to reach the higher notes without struggling so much. It's not the same to sing a song when you record it and you can easily erase and do it again until it's perfect, than when you do it live after having singed a few other songs, specially a band like sum 41 where nearly all their songs are high and difficult to sing. Some people may see that as cheating, but let's remember we are all humans and we get tired, and so do our voices. There is also the fact that he plays guitar as he sings, and that's an extra physical effort you are making so you will get tired more easily. All this facts sometimes make you decide that it's better to play on a lower key. Hope I solved your doubt!:)
