Just for goofs, I plugged in the data from my last century(actually 103 miles.) Finishing time of 6:52:00, average speed of 15.1 mph, but with over 14,000' of climbing.

The "calculator" tells me that I'm only faster than 35% of cyclists my age, calculated average power was only 141 watts, and that my calculated FTP is only 178 watts. In reality, I'm in the top 5% for this course, the actual average power was 210 watts, and my FTP is roughly double the "calculation."

Obviously, the elevation gain is something this app doesn't factor. 210 watts would give quite a higher average speed on a flat century, and I would be able to put out a higher average power on a flat century, as there wouldn't be extended descents.