Alaska polls, at this point in 2016.
President:
Trump (R) 37% (51%) (+14)
Clinton (D) 34% (37%) (+3)
Johnson (L) 10% (6%) (-4)
Senate:
Murkowski (R) 49% (44%) (-5)
Miller (L) 16% (29%) (+13)
Stock (I) 8% (13%) (+5)
Metcalfe (D) 9% (12%) (+3)
There's always one candidate that way overperforms polling. I would expect Dunleavy +5-9% over Begich, or possibly Walker.
Bingo. Third party races are incredibly difficult to poll, and always overestimate one of the party's support.
In that Senate election, the Libertarian and Independent candidates both gained compared to that poll.
And the fact that Walker is the governor could make this a different case than usual. Heck, you could argue that Walker outperformed polling in 2014 (yes, lack of a Democrat, but Kansas was the same scenario but opposite outcome).