Its a goof--his hands are clearly cuffed when he descends into the pit. Kershner instruted the sculpter of the carbonite block that he wanted Han to look like he was in agaony and struggling to break free of the mold--i think he even may have had them redo the original design. So this is why there is the discontiuity. Theres various fan explanations but the truth is that its a simple continuity error. Because its a movie.