I want to say my 2000 C5 has this as well. It's like the whole seat back can twist left and right independent of the seat bottom, at least more then you would like with the outside definitely having more play to push back then the inside.
IIRC I think others who had the problem had to replace the seat to fix, but I could be mistaken. I'll have to go check my seat to see how prominent the problem is currently.
Always thought the seats were flimsy, wish they were like the seats in my G37S sedan!