What is the exact reason 雲, クラウド thinks he's Zack?

well if あなた watch advent children complete the white light is there and zack is standing behind him and 雲, クラウド says "i am your living legacy" then he defeats sephiroth

雲, クラウド thinks hes zack because of sephiroths mind games through the jenova cells thts best i can explain it

あなた need to play Crisis Core to understand that...after 雲, クラウド defeated Sephiroth..he and zack woke up in a laboratory where dr.Mojo planted Sephirots sells to zack and 雲, クラウド and made a new Project...it's kinda weird but u need to play crisis core to understand...anyway excelent game...the whole FINAL ファンタジー 7 :D

*smiles innocently* I have no idea... >:)