Songwriting Credits

[AliP]

Does anyone ever wonder why there weren't more songs that were credited to either "Stevie & Lindsey" or "Lindsey & Christine" etc. For instance, a song like Gypsy. We all know that Lindsey did alot with that song. I know that Stevie wrote it, and as she says, she brings in a "skeleton" of a song and then Lindsey will put his magic in and mold it into its final version. Shouldn't Gypsy be credited to "Lindsey & Stevie" then?

Admittedly, I know nothing about how these things work. Is it because Stevie wrote the lyrics and the simple melody so she is credited with all the songwriting credits? Then I don't understand how a song like "The Chain" is credited to all five members?? I mean, really, doesn't John come up with a bass part for every song, like he did with The Chain?? So shouldn't he be credited on every song? It just seems strange to me. But, then I don't really understand how it works, so hopefully someone can help me out.

Also, compare The Beatles. Why are Lennon & McCartney credited for every song that either one of them did (for the most part anyway, I'm not 100% sure on this.) From what I've read on The Beatles, Paul or John would write a song etc (and I'm assuming came up with the melody etc) so shouldn't only one of them be credited?

Or is it just a matter of personal choice?