What you see here in the top line is a little bit of an arch. That's what you would expect from people who are making clear differences between the word pairs, and that's what you see for people who though they were listening to a Spanish speaker.
La línea de arriba que luce un poco como un arco muestra a quienes captaron las diferencias en los pares de palabras y representa a quienes creyeron escuchar a un hispanohablante.