I have the exact same problem on my my wife's Centurion Turbo. I also thought that the seat post was the wrong size, but it is the same size as Chicago Al's, so I am guessing not.

One thing I noticed is that the both sides of the binder bolt spin on this bike. In most of my bikes, they don't. I have to get a screw driver to hold that side while I tighten the other side with an Allen wrench. Could this be part of my problem? Could the OP have the same problem?

I am not really sure if binder bolts are supposed to work in any sort of special way.