"Hey Bill, we don't want to subsidize rich people's sports cars, what do you think is a good number for defining a sports car that we don't want to cover?

Uh, I don't know, maybe around 60k gets into the sports car range?"

You are right. 60k is an arbitrary number. Just like 50k and 70k is arbitrary.

But they wanted to have a limit, probably so rich people couldn't use it. So they probably just picked a round number that sounded nice.