Pictures not real good but it looks like something got on it, have you tried a good leather cleaner or saddle soap and a small soft bristled brush?
I will try to get a better photo
Back sweat? Use a good leather conditioner like Connolly's or Liquid Leather after cleaning with a good leather cleaner.
I used my photo enhancing program and you can see an angle to the way it reflects the light, I'm wondering if it is sun damage, if so get some leather restorer from Amazon, I had a leather motorcycle seat cover that cost plenty put on and the next year it showed some streaking in the grain, put that stuff on and worked it in and it looked brand new afterwards.
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