Your definition of Native Am. & poll respondents' definition might be different. I.e., anyone born here might consider themselves NA. How many generations back do you need to go to be considered NA?
Oh, I hadn't thought of that. The question didn't ask if someone was native born American. The term Native American (capitalized like that) I just assumed would refer to Native Americans, not simply those of us born in the US without immigrant parents or grandparents... But, you could definitely be right.
Given on her own efforts, US Sen. Elizabeth Warren might be able to direct you on how to investigate this subject if you contact her.
I actually know quite a lot about DNA testing as I've tested and helped several other people with trying to figure out their matches. Talking to Elizabeth Warren wouldn't be helpful to the question I posed as the poll question refers to 100% Native American ancestry. Her ancestry is not 100% so doesn't seem relevant to the question. Many, many, many people in the US have small amounts of Native American ancestry. They would not, however, be fairly described as 100% Native American.
Many people misunderstand ethnicity testing. They think that if, for example, someone tests and gets back a result of no Native American then that means the person doesn't have Native American ancestry. This is a misunderstanding when talking about several generations back. After several generations, for example, it is possible that one is descended from a particular ancestor but has not received any DNA from that ancestor. So, you may be able to say that someone has Native American ancestry to some degree but it is not so simply to say that someone does not (for close relations, yes, it would show up but not more distant ancestors). For example, there is about a 10% chance that one will not share any DNA at all with a proven third cousin. I have a third cousin who also tested as did his mother and his siblings. I share DNA with his mother and siblings. He shares DNA with his mother and siblings. However, he and I fall within that 10% of third cousins that don't share DNA.
Anyway -- I suspect you are right and some poll respondents are interpreting the question differently than I am given the relatively high percentage of people answering 100% Native American...