I find it interesting that your power increases as speed increases. It's the opposite for me and a lot of my teammates. I definitely don't put out as much wattage on a flat route as I do when climbing. Though I know a lot of it has to do with the grade of the road. Sometimes it takes 230+ w just to ride it in your 39x25, other times the grade is less steep and you have the ability to push it up the climb.
I generally don't use the actual calculated average for my rides. By "average", we pick a range to ride in. For example, this time of year my threshold wattage tells me to do longer weekend rides (continued endurance and aerobic capacity) between 120-160 w. This means that for the 2-3 hrs I'm on my bike, every time I look down I should not be under 120 w and also not over 160 w.
After the ride is over then uploading the data to my computer and trainingpeaks.com is generally good to look at what the average power was just to use as a reference. At least for me, that average number isn't used for anything. Power is dependent on wind, route profile (flat, rollers, hilly, climbing), intended intensity, intended length of ride, etc. In other words it's specific.
Hope that helps any.