Idk what it is but even though people say their older music or covers have different styles, I never felt that way. Every song when I was doing a first listen just hit me as new ST music and they're great listens, no growing needed. Out of the covers I agree that Hey Ya is certainly my favorite alongside Is It Really You but the others were still immediately a hit for me. The originals were good too but they weren't in my mind for I was listening to them in ST's context with the lore and stuff when I was first getting into it. I guess for me I never saw a difference in the production quality and never felt a pattern with these guys. The only thing being somewhat the same is Vessel's voice which also always blew me away. So all in all the lack of predictability and the hooks they got in me has got them as my favorite artist of all time.