Different airlines have different pricing policies. Some even have different policies for different routes and even different days of the week or hour of the day. I've seen a previously sold out flight which suddenly had 4 seats available in every class three days before departure except if you booked a seat in any of the available classes all the others went to 3 available. I can also remember being or a really cheap economy ticket coming home from the USA and being plucked for an upgrade to club. I was clean(!) but with a backpack and a 49ers jacket hardly the typical "well dressed" upgrade target everyone believed was the way to go in the '90s and I didn't have "status".
Don't try and second guess airline pricing is my motto!
That said,here's a story from a good few years ago....
Swissair (not Swiss) had a DC10 that came from New York did a quick turn in Zurich and came to London full of "Gnomes" at high fares. I was told they made enough money coming to London that they could have flown back empty. They didn't but it became know as a flight that often offered "reasonable" fares.
Just occasionally if you use lateral thinking there can be clues. But only very occasionally.