Well I didn't say it was necessarily a logical way of looking at it.
Let me try to explain my thinking.
To me, when I hear Life's Been Good, I see Joe performing it way back when. I've heard and seen it so many times done by Joe only. I remember first hearing it on But Seriously Folks and loving it. It will always, always be a Joe song for me no matter how many times I see it performed with the Eagles. It started out a Joe song. Where as One Day At A Time, the first and only time I heard/saw it, was performed by the Eagles with Joe singing lead vocals. That's not to say a year from now after it's on Joe's solo album, without the Eagles backing him up, that I won't feel differently, but for now it's an Eagles song because I've not seen or heard it done any other way.
Does that make any sense at all? Probably not but it's perfectly logical to me.