I wasn't able to read the whole article since I'm not subscribed, but the answer is in the text that was included to me.

It's because "pretty happy" is the 3rd answer to the question and that number has stayed the same.

From the article: “The balance is made up by the pretty-happies at around 57 percent, who I omitted from the chart because their numbers didn’t change from 2018 to 2021.”

There is a third choice. "Pretty happy". It's then middle and left out because it's considered the "neutral" answer