That would be my guess: heat rejection ~ D1^4 ÷ D2^4 in absolute temp, so the difference between 100° F ambient (actual) and 70° F (design) is (460+100)^4 ÷ (460+70)^4, or 983 ÷ 789, or 1.246.
Assuming equal efficiency, you need +25% capacity for the higher temperature.
The reduced air density at Bonneville also reduces efficiency, I would guess in direct propotion to the altitude, so roughly 29.92 (sea level) ÷ 25.64 (4,214 elevation) = 1.167.
If true, the combined factors total about +45%.