Chances are good that you have an "orphan" out near the end of your Timeline.
First, hit the \ [Backslash] key, and study your entire Timeline. Anything out at the end?
Next, zoom in on the Timeline horizontally and hit the End key. Where does the CTI (Current Time Indicator) go? What does the TimeCode show in the Program Monitor?
If you find any "orphans," look at them. Are they maybe a Clip that you forgot to add into your edit? Many can be 1-2 Frame slivers, that got sheared off, and can most likely be Deleted.
PS - welcome to the forum!
Many thanks spot on 1 sliver out in the wilderness
Glad you found the "orphan." Those little waifs can really mess up a production. I've seen many, who Exported to DVD only to find that all of a sudden they needed a DVD-9, when from their intended Duration, they should have had plenty of room on a DVD-5.
Since your "orphan" was a single Frame, what most likely happened was that it got sheared off when you butted up a Clip to another one, with Snap ON, which is probably the best setting for Snap. If one moves the new Clip a bit too forcefully, a 1 - 2 Frame sliver will be lopped off and will go to the end of the Timeline at that point. As one edits and tightens up their Clips, that "orphan" gets left way out in space. The more tightening, the farther out it will be left. When butting up the Clips, go a bit slowly and watch the Edit Line running from the CTI. When it changes over, you have butted the Clips properly, and will not shear off that 1- 2 Frame slice. Gentle is better.
When I get one, it's usually because I populated the Timeline and then began editing and moving Clips around. I've forgotten some smaller Clips and after tightening things up, only discovered my error, when I hit \. After a few times of this, I now make it a habit to look at the entire Timeline, plus zoom-in and hit End, AND also compare my TimeCode for my Duration. I have not had any "orphans" get by me since.
Good luck, and glad that you now have the proper Duration - it's happened to many of us. Thanks for reporting your success.