Peugeot saddle post problem.
 

On my '88 Peugeot Versailles, the very top of the seat tube where the binder bolt goes in is bottomed out. This bike has the original Stronglight saddle post and the seat swivels if i try to ride it. Why would the seat swivel as if the bolt were loose when actually the bolt (and that part of the frame) are bottomed out? Could it be that the post is just a bit too small? I'm thinking of getting a new post that is .1mm bigger diameter.

Loose seat post
 

If the bike was ridden with the seat post not inserted to the minimum insertion point, especially if the seatpost was not inserted beyond the bottom of the slot in the seat tube, the material of the seat tube may be stretched a bit. The other possibility is that the binder was overtightened, and the material stretched.

A bit of filing(be careful not to let filings fall down the seat tube into the BB) on the slot in the seat tube or a small shim from a soda can may solve your problem. It may be difficult to find a seat post in a 1 mm increment.