This is done because the data in Audit is arranged in hourly tables, or monthly tables, or yearly tables.
For example when trying to export a .csv log file from Audit widget, one will have to set the time duration. When the time duration is set to hourly, it rounds off all log lines to hours eg 1 pm, 2 pm, 3 pm, 4 pm, etc. When the time duration is set to daily, it rounds it off to an hour each day. Such as 4.00 pm each day. Similarly, if the time duration is set to monthly, it rounds off values to one specific day of the month. This rounding off procedure is referred to as mapping.