The legs on the ESB one are also substantially taller. They're the same basic shape, but the ROTJ legs are substantially shorter.
I believe it was because the original model partially melted after being left under bright, hot studio lights during ESB's shoot, so the parts that were melted were re-done from scratch for ROTJ. It's possible that that's a load of dingo's kidneys, but it seems plausible enough to me.