You could have a grandparent who was 100% native american. Your parent from that branch would be close to 50% native DNA but there is no guarantee that you would have any native american DNA, you might and might not, you could have sibling with a large percentage while you have little or none. Before I got the test I mistakenly believed the DNA was split evenly from you ancestors, not true, its pretty random.
I think you're right Joe, done a lot of reading on the subject when a family member got tested with surprising results. Way over my head, but I do remember some genetics from ol' Mendel and his peas. The family member's grandfather married a Southern Ute woman, we have the documents and entry in family bible. Great grandson shows no native blood. Unless genetic research is done for a legal case, I wonder how accurate these tests are. I mean, let's be honest, it's not like we can run our own test to check on their work. Hire another company maybe, but how would you know if one or the other was accurate...or neither one?
