Hmm, I'm obviously not the best source of accuracy on this question, but based on interviews and such, it's known that the songs are based on life. their own lives and the lives of those close to them (when they hear a story or situation, they write about it). I believe I read that the song "Stately Lover" was about a friend of the band's...not any one of them in particular, so that could be one answer.
Also, Cristina has said in interviews that when they write, being in a melancholy mood tends to be best because if they're in a happy mood, they'd rather be out with friends having a good time, not locking themselves up and writing about how happy they are.
I would say it's safe to assume that many of the lyrics are based on memories. But that's just me.