Some of it, is, of course, 12 summers in the sun. It's quite possible, as Ben has discussed the extreme variability of vinyl quality, that this vinyl was less impervious to UV rays. The floor of the liner faded far more than that walls of it (you can only see a little on the right of the picture) which leads me to believe UV more then FC faded it. That wall portion you see is on the South side of the pool, so always a little more shaded as it looks North. Some is due to my preferring to kill a bloom fast, so we can swim faster, such as when we came home from a month away to a green pool, knowing I was planning to change out the walls and liner, I simply hammered the FC (and replaced the coffee maker! )

I suspect that over a decade most liners fade and fade a lot, but if you don't have a reference point, as I do, you only sort of notice it.