Figures were just for discussion sake.

Some fates, simply give less. If they do give less then others in the area, it's also quite possibly a part of a chain of fates. And obviously the large boss fates which whilst could be at the same level as other fates in the area is going to give more.

As far as I was aware, bronze, silver and gold xp is fixed, as in there isn't an xp threshold. So if you get gold, that's it, there's no way to get a better gold and thus more xp. But they are affected by your level and any xp boosts you have active.