This is because settings have been changed or more cameras have been added.
For example, if your DVR is set to 'always record' then this will give you the shortest recorded time period.
If you select an option such as 'motion detection', this will give you a longer recording period as less data is being stored.
The same applies to the number of cameras used on the system. An increase in cameras may reduce the recording time period due to an increased amount of data being stored.