Somebody posted a theory saying that Avril Lavigne commited suicide in 2003 and was replaced with a double. I saw this theory both in English and Spanish, but now I could only find it in Spanish
here. To sum it up, it says the "New Avril" was giving hints on her following albums about that replacement, with the music, lyrics and booklet arts; Under My Skin would serve as "mourning album", paying respect to the "Old Avril" and explaining what happened. Later, The Best Damn Thing would show the definitive change between both "old" and "new" Avril. It also shows photos from all her eras to show they are two different people.