Yes, you are right. lightdm's config file says to use both unity-greeter and a session of Unity 3D (definitely set wrong)... On Xubuntu!?!?! ...But lightdm is still making a call to startxfce (2nd to last line in your lightdm.log) from somewhere and may be as confused as we are??? Something is definitely not right or different in this. As Spock would say: It is not logical.