The subsidy is set up so that if you're within the 133-400% of poverty level range, the net cost of the second-cheapest "silver" level plan in your area after subsidies will be a certain percentage of your income. People who make 133% of the poverty level are expected to pay 2% of their income for that plan, while people who make 400% of the poverty level are expected to pay 9.5% of their income. There's a sliding income scale for people whose income falls between those extremes. If the second-cheapest silver plan is more expensive than the set percentage of your income, a subsidy pays for the rest. The subsidy's value is set based on the cost of the second-cheapest silver plan in your area, but you can use that subsidy amount for any health plan offered through the exchange.
In Missouri, it looks like the second-cheapest silver level health plan for your hypothetical family must have had an annual premium of about $8,088. Your hypothetical family with an income of $83,603 qualifies for a $146 subsidy because the second-cheapest silver plan for their family costs $146 more than 9.5% of their income. 9.5% of the difference between $84,780 and $83,603 is $112, which is why the higher-earning family's subsidy is $112 lower. $8,088 is 9.5% of $85,136, so a family making more than this would not be eligible for a subsidy.
In California, the premium for the second-cheapest silver plan must be higher ($10,544 per year, by the look of it). A family with an income of $94,200 would be expected to pay 9.5% of their income ($8,949), and the remainder of the premium would be subsidized. But the law authorizes no subsidies for people making more than 400% of the poverty limit, so there is a big cliff in the tax structure that people need to watch out for here. The family in Missouri wasn't affected by it because their insurance premiums are low enough for the subsidy to phase out below 400% of the poverty level. People in more-expensive states (or older people whose premiums are more likely to exceed 9.5% of 400% of the poverty limit) need to take care to avoid this cliff.