It's entirely possible that one or both of your parents have a Polynesian ancestor. It could also be that Ancestry's "Polynesian" category is not that accurate and is causing some confusion.

I'm part-Hawaiian and have found Ancestry's "Polynesian" category is a little confusing. I've discussed it with other folks in different forums, and several people of Filipino and other western Pacific ancestries have said Ancestry put them in the Polynesian category. Ancestry gives a somewhat vague explanation - that Polynesian is "also found in" areas outside of traditional Polynesia.

Ancestry added the Polynesian category last year, when they revamped their categories and calculations. Before that, people of Polynesian ancestry would have to look for the presence of both East Asian and Pacific Islander in their results. And actually, that's how it still is on other services like FTDNA and 23andMe. FTDNA uses East Asian + Oceanian, and 23andMe uses (a more specific) Southeast Asian + Oceanian.

Also, the Pacific Islander/Oceanian categories generally include DNA from Papua New Guinea and Melanesia. It is generally believed that Polynesians are descended from people who migrated from Southeastern Asia, through the western Pacific/Melanesia and beyond (which is why that East Asian/Southeast Asian + Oceanian combo makes sense).

It might help shed more light on your results if you submit them to GEDMatch.com. You'll get more detailed ethnicity reports and can compare with others who have submitted their results to GEDMatch as well.