Fixed in ArcGIS Online 6.2. As a result, customers may see a spike in their credits for storage between July 3 and 4 due to the reporting showing the correct number.
Understanding Irregularities with Hourly Feature Storage Charges Related to Bug-000100343, https://community.esri.com/groups/technical-support/blog/2018/07/23/understanding-irregularities-with-hourly-feature-storage-charges-related-to-bug