Even after personally serving the three uninvited guests, why did Dev fail to recognize that it was Sai Baba?

Despite personally serving the Sanyasi and his two companions, Dev failed to recognize them as Baba due to a "veil of Maya" that clouded his perception. Chapter 40 explains that his mind was fixed on the simple idea that "He is some guest who has come to get a meal." This state of delusion was so strong that even after the direct experience of serving three uninvited guests, doubt remained in his mind. He was expecting a different kind of experience or proof and was unable to see the divine in the ordinary guise of a Sanyasi, ultimately leading him to seek confirmation from Jog because he couldn't trust his own experience.