How does Megatron's arm cannon reassemble in Transformers One

During the battle between Optimus Prime and Megatron, Optimus forcibly ripped off Megatron’s cannon and struck his chest with it. But, the artillery was later seen mounted on Megatron’s back. Then, as the fight neared its end, the cannon returned to Megatron’s right arm, and Optimus cleaved it using the Energon Axe. What are your thoughts on this?
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