But we're not discussing every other aspect of life. We're discussing tax rates. And tax rates are discussed relative to earned income. Why? Because that's what they're actually calculated against, not other tax rates.
If you save 15% on car insurance, it's relative to what you paid on car insurance. But car insurance isn't based on some 3rd variable. Tax rates are. Tax rates are based on income. Car insurance is only based on other car insurance rates.
You don't just pick some random number out of the sky and hand it to the government for taxes.
Your cost of a coat example is actually not bad. Macy's sets a price for a coat ($100), then gives you 25% off ($25). If Macy's offers you another 10% off, is it based on the original purchase price or on the 25% off that you already got? They yield very different numbers.
If it's 10% off the 25% then it's 10% of $25, $2.50. Given that the original cost of the coat was $100, it's a 2.5% reduction. If it's 10% off the price of the coat, it would be a $10 reduction. That means the difference between paying $72.50 (27.5% discount) and paying $65 (35% discount) for the same coat.
To any person who's buying the coat, they wouldn't equate 10% off the 25% as the same as 10% off the original purchase price. They would, rightly, call the $2.50 reduction as a 2.5% reduction on the price of the coat, not as a 10% reduction on the price of the coat.
In tax terms, your earned income is the price of the coat. You calculate your savings against that price, not against the relative value your other coupons.