I see a lot of people reporting the number but I don't see an original source in any of the articles. Do you know of one?
Would be good to know who they polled and, more specifically, where they polled them. I imagine that Navajo in the southwest have a different view than Shawnee in Ohio, for example. And, of course, I imagine Powhatan have a different view than Chinook. Might be good to know if they polled on a reservation versus in a city. Only 20-ish percent of Indians live on reservations and I imagine one's viewpoint when assimilated with a much higher number of culturally identical people would be different than one's when living in a diverse neighborhood in Phoenix, for example.
Another hunch, which makes a little bit of sense - since the term "redskin" originates, at least in its current form, from the body paint used by the Delaware (literally red skin), and Delaware today only constitute 16,000 of the population according to Wikipedia (75% living in Oklahoma; don't know the reservation), the idea that only 10% are offended is probably because most aren't familiar with the term in that sense. I imagine that number will change once the concept becomes more widely known. A poll in five years, for example, may turn up a much different number.