South European? Do you mean South American? The former isn't related to being Hispanic.
I wouldn't worry about the Census. It's once every 10 years anyway. And it's about self-identification. The question is, does the information change your self-identity?
It very well might. But you need to verify it first. Ancestry's ancestry information is notoriously bad. If you test elsewhere and get the same results (or if you take your raw data and plug it into the awesome ancestry tools at gedmatch.com, assuming you have raw data), that's step one. Or if you take this and go have a chat with some of your older relatives and find out some family stories, that's step two (or co-step one, your choice).
So go ahead and explore it. With 20% that means you probably have a grandparent who identified as Hispanic or Latin American or Indian. Or a couple of great-grandparents. Unless there is an adoption or non-paternal event in your past, this is information someone in the family may know. Or things you can discover with documents.
So how does it change your view of yourself and your family? That's what matters.
Have fun with it and don't worry about what to say on government forms.