The data for September 17, 2021, includes multiple days of reporting from Florida. Florida only reports COVID-19 data once per week via their COVID-19 Weekly Situation Report, and does not provide a daily back-distribution of Case or Death data. The CDC also supplies some daily reporting for Florida. While not all of Florida's Cases/Deaths were diagnosed/occurred on September 17, we are unable to break them out to dates within the week. This contributes to the unusually large number of daily cases for September 17 in the US.
Reduced counts in U.S. cases and deaths are the result of states and territories not reporting the information for some or all of the weekend. Those states and territories are: Alaska, Colorado, Connecticut, District of Columbia, Florida, Georgia, Guam, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Minnesota, Mississippi, Montana, Nebraska, Nevada, New Hampshire, New Mexico, North Carolina, Northern Mariana Islands, Oklahoma, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, U.S. Virgin Islands, Virginia, Washington, West Virginia, Wisconsin, and Wyoming. Typically, these states' Monday updates include the weekend totals.