* He's a perfectionist: Maybe Igor is so focused on getting perfect scores that even a perfect score isn't enough. He might be upset that he didn't somehow get extra credit or an even higher grade.
* He expected a challenge: Igor might have been hoping for a harder spelling test. He enjoys the challenge of learning new words and feels like he didn't get to stretch his skills.
* He's feeling pressured: Perhaps Igor is feeling pressure from someone else to get a perfect score. Maybe his parents or teacher have very high expectations, and even though he did well, he feels like he let them down.
Without more context, it's hard to say for sure! Let me know if you want to add more clues to the riddle, and I can help you solve it!