Part of the reason for the discrepancy between the Gallup estimate of consumer spending and the Commerce Department estimate of retail sales might have to do with underreporting of online sales.
According to Commerce, online sales grew by 14% last year. Amazon revenues grew by 22.4%. That’s a big difference, and it isn’t clear why the revenue growth at the largest online retailer should differ so widely from overall online sales. Amazon accounted for roughly half of all online sales during 2016, so overall sales growth should be roughly comparable to Amazon sales growth.
If retail sales really are underreported by the Commerce Department, the US economy is growing considerably faster than most economists believe.