Factual error: During most of the close action shots of Perseus flying on Pegasus, you should see some wing expansion, but do not - only during the wide pan claymation shots.
Factual error: Perseus and his mother would suffocate in the box within minutes. There were no openings for air.
Answer: To me, it just looks like the way the prop catches the light as the actor lifts it up, and then higher up, the angle of the prop isn't catching the light in the same way.
Bishop73