Duration getting updated to inaccurate number when the tasks are assigned to an iteration

Hi All, 

I have a project created with a set of tasks. I had updated duration, planned hours and assigned the tasks to an agile team. Once the tasks were assigned to the agile team, those tasks were showing up in the team's backlog. When I added few tasks from the backlog to one of the iterations, the duration of the tasks in the tasks list in the Project page got updated to very large numbers (57 days) whereas the original duration i had updated is very less ( 1 Day). Not sure why this is happening. How to retain the original duration of the task after it is getting assigned to an iteration.

Please provide your inputs. Thanks in advance.