I am studying the BB84 protocol and I think I have a slight misunderstanding. As far as I understand, if Bob measures the photon in the same basis as it was prepared by Alice, he is going to receive the exact bit Alice intended to send him without any randomness. If Alice and Bob have a basis mismatch, then bits received by Bob are going to be random.
Now consider this image I got from an article on the same subject at
Lets take the third bit 0. Here A prepares the photon in diagonal basis and Bob measures it in the same basis. Yet the bit Bob records is different from the one Alice sends. I do not understand how. I would really appreciate someone telling me where exactly I am misunderstanding the protocol.