My problem, like the other Bay Areans, is hills. I've been doing a similar thing - I've got a 25.5 mile route I like to ride midweek, but it's got about 1100 feet of climbing - and I consider it my 'flat route'! The best I've managed is 18.8 mph, and the worst is 17.5, and the difference definitely comes down to the bike. Some bikes are just faster, given the same engine.

EDIT: The other thing? Wind. Because I ride after 5PM, I'm fighting the wind a lot. Mostly it seems like places where the wind direction would help are protected by trees and such, whereas places where it's in my face are wide open. And it increases during the ride so that any advantage I'd gain heading South at the start is more than compensated by a stronger headwind when I'm coming back North.

Yeah - THAT'S why I can't crack 19mph. Nothing to do with my own ability or fitness.