if sahir is

shown as that smart that he immediately guessed that arzoo came to know the it was him who had done the factory blast then how come he did not remember anu's name when zeenat had said and whatever the secret is would have been shown now. And after seven years he remembered that zeenat had mentioned someone called anu is strange considering he is the person who never forgets anything.

The problem with your question has logic. So no explanation... End of story.