Shutter speed changes automatically for different shots?

I'm currently shooting a short with a Nikon D3100. I shoot wide open with a shutter of 1/50 and have the camera set to manual mode for obvious reasons. I'm now looking back at the previous scenes I've shot and I noticed that some shots have motion blur (which is expected from 1/50 shutter) and other shots in the same exact scene have absolutely no motion blur at all. I'm looking at a person move their hand and there's motion blur followed by another shot where someone is moving their hand with no motion blur AT ALL. It looks like I shot it with completely different settings which makes no sense at all because I ALWAYS have it on manual with 1/50 and f1.8. Is this a common problem? Is there a factor I'm not taking into account? Is the lighting screwing with me? Is the camera mentally slow? I'm so confused right now and I was hoping someone might have a clue.

Added (1). It looks like as if I'm shooting 1/250 shutter when it clearly says 1/50.

Ensure lighting is constant so exposure does not change. No, sun or fluorescent bulbs. Use other types of continuous lighting system.

The speed of the motion is not consistent.
The blurred hand moved faster than the one with no blur.
Lighting has nothing to do with it unless you use a flash.
If you want more blur at that shutter speed your subject needs to move faster.

Check the metadata on each photo to see what shutter speed was actually used for each photo (File Info, or Properties).

If you truly had your camera on Manual (also check in metadata), then the camera made no alteration - that is what "Manual" means, and if you pics are under or over exposed that is your look out. With an exposure level of f1.8 at 1/50th I assume you are working in fairly dim lighting if you get correctly exposed pictures. As Fhotoace says, the speed at which you move your hands is inconsistent (as it is for all humans), thus some pics will be sharp at 1/50th, and some will be blurred.

I don't think setting manual shutter speed and aperture for videos on the D3100 is quite as simple as you imply. Normally, it's not possible in a general sense but can be done if you follow the right procedure, which is outlined in the following video: