My dad had a similar experience. He's taken several different DNA tests, and all of them give him different results. His great-grandfather apparently had an affair with a Native American women, who had a baby but didn't want it, so his great grandparents took the child in and raised it as their own, only revealing the truth to him later in life.

My dad has taken several tests. One said his DNA is 10% East Asian, another said he had no East Asian DNA but that he was ~8% Middle Eastern and ~1% African. He's currently waiting for his 23andMe results, but mine said I was only 0.5% Native American and that the remainder was European, so I'm guessing it'll be similar for him.