I guess they happened to have a fake head of George Bush in the props department? Maybe that's where the Phil Dick android head is, too!
According to the creators of Game of Thrones' DVD commentary, the head of ex-POTUS George W. Bush made an appearance on a spike in the show's 10th episode from the first season.
Creators David Benioff and D.B. Weiss explain "it’s not a choice, it’s not a political statement. We just had to use whatever head we had around."