I received a response to my inquiry to the EIA last week. Glad to see they are surprised by the numbers as well.
Thank you for your interest in our data.
I work on the team that puts together the weekly publication. We understand that many were surprised by the four-week average finished gasoline product supply dipping below the 2020 four-week average finished gasoline product supply. There are a couple things at play here. As you mentioned product supplied is a measure of the barrels of gasoline as the leave the primary supplied chain to the secondary or retail supply chain and therefore does not represent actual barrels consumed by drivers. If gas stations are well stocked on gasoline inventories they may elect to receive a lighter than normal shipment from nearby bulk terminals and that would look like decreased product supplied even though drivers continue to purchase gasoline.
Our product supplied calculation does include imports. Product supplied is calculated by adding the refiner and blender net production, adding the amount of finished gasoline imported, subtracting the amount of finished gasoline exported, subtracting the amount of gasoline put into inventories/adding barrels of gasoline pulled from inventories, and adding a balance adjustment for ethanol and motor gasoline blending components (MGBC). The balance adjustment is reported separately in our tables and represents a balancing item for the over or under supply of ethanol and MGBC that may arise from timing or survey error. More information about product supplied and balancing adjustment can be found in our explanatory notes for the Weekly Petroleum Status Report.
I, like many, was surprised to see finished gasoline product supply down this week but this is what our survey data was telling us. Survey processing and data quality work proceeded as normal and survey response rates were high as they normally are. There were no obvious abnormalities in their respondent level data. We will have to see if what the much more definitive Petroleum Supply Monthly will have for July when it is released in October. Usually the two series track very closely and are within 5% of each other.
Hope that helps.