Although you get 25% increase of reputations gained with Nightfall buff the maximum you can get from doing lvl 32 weekly heroic is 499. Despite the fact that 25% of 400 is 100. So 400 +100 = 499 is Bungie's mind.
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Floating point (non real numbers) aren't exact when it comes to machines. The best you usually hope for is a very close approximation. Combine this with integers being rounded down automatically in most programming languages and suddenly you have 499 instead of 500. It's easy to let something that small slip through. Source: I'm a programmer.
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Dumbass Bungie employees, can't live with them.