Using technically impossible shutter speed when shooting video
Whether this happens inside your specific camera is not clear. However, it is possible to have a shutter speed slower than the FPS would naively allow. In practice, it's very unlikely that a camera would do this if uses a mechanical shutter for video, but it can be practical in electronic shutter cameras.

Imagine a shutter speed of 0.5 seconds, and a framerate of 4fps. Each 'exposure' lasts 0.5 seconds, but each frame's exposure overlaps with neighbouring frames - the frames are not the same, but they overlap. 

Imagine:


A is the light value that arrives between 0 and 0.25 s
B is the light value that arrives between 0.25s and 0.5s
C is the light value that arrives between 0.5s and 0.75s
D is the light value that arrives between 0.75s and 1s
etc...


Then:


Frame 1 contains the light A+B
Frame 2 contains the light B+C
Frame 3 contains the light C+D
etc....