it was an advance! Condensed from Snopes:
The tax cut resulted in us paying $300 (or whatever) less in income tax than we would have without the cut. In other words, the $300 (or whatever) would have come next spring anyway. Thus, it’s not a refund from last year, it’s an advance on this year. Plus it expires in 10 years, then we’re back where we were with out the cut.