I think a few readers have forgotten Jun is actually smart. He was just romantically super dense. However, his grades have always been good, unlike Tomo's. So, once he got over his romantic density, he could naturally put his wits to use and talk like this. There's nothing strange about it.
I compete with my fiancée all the time. We get pretty intense. Romance and competition are not mutually exclusive. In fact I would argue that it makes our relationship better. More fun than it would be otherwise.
@Closedbox We don't compete about that. The way we use make up is completely different anyways. I enhance my cuteness and she just uses lipstick and eye shadow. Although we do compete about coffee, tea, and candy. Little but important things