There are a number of reasons you might be seeing this:
1.Your video is not playing in full.
2.Your video is repeating itself.
3.You need to adjust the speed of the video.
You will get these options on the timing section if your media is a video:
Please note in the green coloured section that the total video is 11:04secs but the scene time is set to 1:02secs. If you want the video to play in full, please (right check) the 'Play Full' option.
The 'Loop' option will be used when you want the Scene to be of 15secs in total but video is only 11:04secs. If you (right check) 'Loop'- the rest 3.96 seconds the same video will repeat.
The 'Playback Speed' can be used to show special effects such as 'Slow Motion' or 'Time-lapse' in the video.
The total time of the scene will always be equal to the end-time of the last caption.