When you enter a start date, you are creating a Start No Earlier Than constraint. If you then enter a finish date, you are setting a Finish No Earlier than constraint and the Start No Earlier Than constraint is removed. If you chance the Start date again, the duration should drop or increase. However, if you change the Finish date, it will maintain the FNET constraint and keep the same duration.

Why are you manually entering start and finish dates for tasks? You should enter the Project start date, the duration (or work) for each task, and link the tasks. If you are going to enter all the dates, either use manually scheduled tasks (Project 2010 and later) or consider using Excel. Project's purpose is to calculate schedules.