The Pshat is like this. In Yevamos 106a, even though Abaye caused Rav Papa's parents to die, the Ya'avetz there (p. 64 at the back of the Gemara, after the Rashash) writes that this was not deliberate, Chas v'Shalom. Rather, what happened was that Rav Papa asked Abaye some strong questions and as a result Abaye asked him back where his father was and where his mother was. Rashi DH Avuch writes that Abaye felt that since Rav Papa was so sharp in his learning this suggested he probably had parents who were supplying all his needs, which kept his mind free for learning.

Aruch l'Ner explains further that Abaye actually felt that Rav Papa had understood this Din better than he had himself. He felt that this must be because he himself was an orphan (see Kidushin 31b) whilst in contrast Rav Papa was helped by his parents. (See also Eruvin 65a where Abaye remarked that even if he had to perform some minor chore this had a negative effect on his ability to study at his optimal level, so this explains further his admiration for Rav Papa and his parents who must be helping him so much.)

Ya'avetz explains that it was this admiration for Rav Papa and his parents that inadvertently caused "Ayin Ha'Ra". (See also Kehilas Ya'akov Bava Kama 45:4 DH Amnam who gives a similar explanation to Ya'avetz.)

Now we can understand, BS'D, the Gemara in Kesubos 17b. In this case Abaye was disapointed that Rav Papa did not know the prevalent custom. His reaction was now the opposite to his reaction in Yevamos. Whilst in Yevamos he ascribed Rav Papa's success to the help of his parents, in Kesuvos he remarked that the reason Rav Papa was lacking in his knowledge must be because his parents had now died (i.e. it seems that the account in Kesuvos occured chronologically after the incident in Yevamos), and he longer possessed the utmost level that he previously had.

Yeyasher Koach for raising the question.

Dovid Bloom