I have to grind up the hills on the race bike, and I spin up in granny gear on the Surly. I ride at a fairly fast pace.
It's because of this. As an engineer you'll realize that that your total time for the trip is dominated by the slowest speeds, not the fastest. Since your hills are relatively slow ("grinding up", "granny gear") compared to your faster pace, the top end speed is almost irrelevant.
Given a lesser grade on hills or more "oomph" in climbing I'd bet that you'd see a much bigger difference using your faster, lighter road bike.