User wants to know why the Client Hours field for time entries always defaults to a particular value, even if they are being changed to another value.
This can happen if you set default minimum hours that employees have to work on an activity in the Activity Items or Fee Schedules screen. You can set a minimum number of hours to work on an activity in Activity Items > General in the Min Hours field. Check the Core Help for details. Similarly, you can set this in Fee Schedules Services > Edit Service Fee Line. Check the Core Help for details.
In both cases, the corresponding time entries do not take any value for Client Hours less than this minimum hours. You can change it to a higher value though. Check out this article for more information: Client hours are not updated along with actual hours.