This is because the data is arranged in hourly tables, or monthly tables, or yearly tables.
For example when trying to export csv log file from Audit widget. One will have to set the time duration. When the time duration is set to hourly, for example, it rounds off all log lines to hours, eg 1 pm, 2 pm, 3 pm, 4 pm, etc (so 24 values). When the time duration is set to daily, it rounds it off to an hour each day, so let's say 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 was referred to as mapping.