Sreeram,
Which version of PPM are you using? I am checking in 15.4 and in the new UX I do not see an option to copy hours.
In Classic the options there are Populate assigned tasks and
Copy time entries from previous timesheet. On this page you have to ability to change the Display Unit from Hours to Days.
In the New UX timesheets it does say Days on the timesheet vs Hours when I have the System Option set to Days.
'Task List view, each Top node(Master of WBS) show the Actuals in Hours' is this the new UX? I do not see this configuration on the Task List in the New UX.
Perhaps you should enter a case for Support so this can be looked at?