Occasional feline skin twitching is a signal that a cat dislikes something that is happening at that time. It can be a signal that a cat disagrees with what is going on and/or he or she is uncertain. It can also signal anticipation. In general it is a response to an emotion.
The fact that the cat and other animals can twitch their skin means there are muscles under the skin that are specifically designed to move it. I believe it is a sheet of muscle called the cuteness trunci muscle (CTM).
The evolutionary reason for the existence of the muscle is probably to disturb flies and insects on the skin/fur. It is a reflex action. This has been hijacked by an emotion.
I have decided that occasional feline muscle twitching is an emotional/psychological response based on first hand observation. The observation is mine and it does not come out of a book or webpage. If anyone can find a source of information about this, other than a reference to feline hyperesthesia, please leave a comment. It is very hard to find solid information on this. Incidentally, uncertainty is signalled buy a horizontal swishing of the tail.
This cause – an emotional response firing off a reflex action – is to be differentiated from feline hyperesthesia, which is a medical condition, the cause of which is unclear (probably stress). Perhaps the two are linked.
Horses also skin twitch and it applies to other animals too. In horses, the skin twitch is often used to get rid of flies. However, the reason can be psychological (fear, anxiety, uncertainty etc.), in my opinion, as for cats.
When people twitch it is usually the contraction of muscles that control a piece of anatomy other than the skin. A different set of muscles are involved. Although, for example, a twitch under the eye seems to have similar causes: an emotion.