Duration units and due date calculations?

When creating a task with work = 1 hr and a duration of 1day the due date is the Start date +1 and in other projects a similar task the due date = the start date?  Are there any settings I may check to be consistent when calculating my due dates?

Also, when changing the during to units from days to hours in the Start date + 1 scenario, I have to have a duration of at least 2hrs until the task due date = the start date.

Any help understanding the differences is greatly appreciated.

Thanks,

Stalin