It's just the way Ancestry breaks it down. Even their Polynesian category casts a broad net, including Pacific Island peoples who aren't typically considered Polynesian.
You might get more detail if you export your "raw data" from Ancestry and upload it to GEDMatch. (
http://gedmatch.com)
I know it's possible to break-down East Asia with more detail. For example, 23andMe offers a more detailed breakdown in its East Asian category: Chinese, Japanese, Korean, Yakut and Mongolian.