The duration is based on days used and not days left.
So most likely because you have it listed longer than 10 days.
To change a fixed price listing duration requires that you have that option left in the listing.
A fixed price listing has a duration of 3,5,7,10 and 30 days. When you first list a Fixed Price 30 day listing, you can change to any of the durations. After 3 full days (3 x 24 hrs), the 3 day option disappears leaving you with 5,7,10 and 30. After 5 days the 5 day option disappears leaving you with 7,10, and 30 days and so on.
So I will speculate that your listing that cannot be revised is past 10 days.