Under the Single Mailing Report summary, the data is sometimes displayed by hour, date, or week. The graph and rows of data that is generated for the Single Mailing Reports - Summary works on a set of logic that requires no more than 24 data points to exist (on either the graph's x-axis or how many rows of records are in the tabular data). Meaning, if an email has activity over the course of less than 24 hours, the graph and data can show a true hourly breakdown.
As activity extends over beyond 24 hours, the graph and tabular data aggregate bi-hourly. So, it represents time as such: 10AM-12AM, 12PM-2PM, 2PM-4PM. This aggregation lasts through two days of activity. The process repeats for the third day. The graph and data starts breaking down the activity into three-hour increments (e. g. 1PM-4PM, 4PM-7PM) so the number of data points does not exceed 24.By the time a email has activity to record (defined as opens/clicks) for more than 72 hours, the report starts producing a daily breakdown. The product was designed with this sort of logic so the report can dynamically generate the most helpful graph and set of data at the time of report creation.