It's quite simple actually. The marginal tax rate on the first 10,000 of income has been lowered from 15% to 10%.
So when calculating your total income tax you have to factor in the rate reduction that was passed as part of the overall long term tax cuts that were adopted last year.
And for the next several years, as the tax cuts really kick in and start to affect the higher tax brackets, your refunds will shrink even more.
By the way, a couple of statistics, only 5% of the workers in the US earn $110,000 of taxable income or more, but that 5% pays 60% (sixty) percent of the personal income taxes to the US treasury !
If you earn $36,000 or less of taxable income, you are in the bottom 50% of US wage earners and you are part of the group that pays only 4% (four) of the total personal income taxes.