I can always tell when the people commenting have never ridden for hours at a time in the mountains. The big difference is that the power requirement to keep moving forward is unrelenting. On flatter terrain, you might put out the same wattage, most of the time, but it's common to relax once in awhile. That relaxation period often goes unnoticed. Try that on a big climb and you'll be going backwards.
There really is a big difference between training in the mountains, compared to training on the flats. You need to train in the mountains to be proficient at climbing.
Body weight can be a huge issue. Heavy riders can ride well on the flats, where wind resistance is the major obstacle. Put those guys on a hill or mountain and a 20 mph average guy will ride at 8-9 mph.