How is it possible that Claudio was deceived? In other words, how was the tryst interpreted and by whom?

Both Don Pedro and Claudio witness Margaret, not having any idea about Borachio's evil intentions, at Hero's window. Borachio will speak "love talk" to her while Don Pedro and Claudio witness from a distance. Both think Hero is a strumpet and that he (Claudio) is about to marry a "contaminated stale." Claudio does not attempt to investigate further. He takes what he thinks he sees from a distance at face value.