This is a common phenomenon for most things because the people in the lowest intake category of a thing are normally there for a very specific reason.

For example, people who don't drink any alcohol are more likely to die than people who drink a little bit. Why? Because people who don't drink at all tend to be those with liver damage or former alcoholics who have already done permanent damage.

I suspect a similar explanation here. People who avoid added sugar probably have other health complications going on that force them into the lowest intake category.