The question is what average; some people apparently view "in the US" as implying US population-level averages (which it does not explicitly imply), whereas authors report the average within adopting households, which for this study's data source, all happen to be in the US

If the claim was just that grocery spending is down 5.3% across the country they wouldnt have said average, the title would just be "Ozempic reduced grocery spending by 5.3% in the US"