https://www.cdc.gov/mmwr/volumes/70/wr/mm7023a3.htm

“Because sequences might be generated by multiple laboratories and sequence availability varies both geographically and over time, **CDC developed statistical weighting and variance estimation methods to generate population-based estimates of the proportions of identified variants among SARS-CoV-2 infections circulating **nationwide and in each of the 10 U.S. Department of Health and Human Services (HHS) geographic regions.*”

CDC admission the Delta Variant "outbreak" is not a measure of sick patients confirmed positive for this particular strain, but a statistical model.

If you follow links on the CDC website, these are non peer-reviewed classifications and definitions: https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-info.html