I think you’re missing what an “effective tax rate” is. If your country’s income tax is 0% on the first 10k, 10% on the next 10k, and 20% after that and you earn 40k, your effective tax rate is:

• 0 on the first 10k
• 1k on the next 10k
• 4k on the remaining 20k
• 5k total out of 40k = 12.5% effective tax rate

The wiki page does not show that data.