[O_H_E]

Note: I am note defending any previous arguments, just trying to make a fact clearer.

Your calculations was right in that idealistic tax system, but the real world is really messy with lots of exceptions, Tax credits, Benefits, and such.

Building on your example, I will add a little Child Benefit to make a slightly less ideal idealistic-scenario.

Assume If earning < 100: child_credit = 3

Assume 1 Child

If earning 99: take_home = (99 * 0.9) + 3 = 92.1

If earning 100: take_home = (100 * 0.9 + 1 * 0.8) = 90.8

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I chose this example because I remember vividly a story about a family in the UK that avoided getting promoted because their take-home would decrease. Of course it have to be a very small promotion in order to not be worth it. I don't know if cases like this is rare in the US, but it is generally a thing.