Either he was fighting on the battlefield and was preparing a chidori until the guy moved out the way and was getting ready to hit his enemy until he realized it was too late to stop his chidori and struck rin.
Or zetsu purposely altered obitos view of what really happened that moment either through genjutsu or other means because zetsu probably realized that tobi will need to forget his good ties and be evil.
Or zetsu purposely altered obitos view of what really happened that moment either through genjutsu or other means because zetsu probably realized that tobi will need to forget his good ties and be evil.