If the timeline duration is not the same as the exported video, then there might be some small fragments left on timeline. You need to locate the fragments and then manually remove them. You could follow below steps to locate and remove them.
1. Press Zoom to Fit Timline button to show all the contents on timeline. Then find the fragments and remove it manually.
2. If you still could not locate the fragments, please press the Zoom in button to enlarge the timeline to the maximum. Right click the fragments and choose Delete.
3. After delete the fragments, you will find the duration time has changed in the preview window. Meanwhile you could replay the project file again and see if there is extra blank frame.