My guess would be because sats are not perfect spheres but - esp in the case of a rocket stage like here - complex objects which have non-trivial rotation patte

My guess would be because sats are not perfect spheres but - esp in the case of a rocket stage like here - complex objects which have non-trivial rotation patterns because the center of gravity might be off the visible center. So I doubt they model each and every piece of space junk out there to get a super detailed prediction - after all, what can you do if you know 95% it's going to be a hit between do dead sats vs if you know 80%. Radar resolution of civil orgs might also be a limit.