Like Hopingforcure said, my neurologist told me that although they (the medical community) do not really understand why fasciculations happen, that one of the reasons may be nerve irritation.
In my own case, I've not ever been very twitchy. With bulbar onset I had clinical weakness noted by various medical professionals that I either was not consciously aware of, or did not realize the extent of, and had not felt any fasciculations in those areas. Now that there is limb involvement, my left hand has some visible muscle loss, also is weak (and my wrist) but only once was I aware of fasciculations there, and that one episode was one of an almost continual more intense movement, as opposed to the ongoing slower more gentle ones that I have in my feet and legs. But both places have atrophy.
Fasciculations in a specific muscle does not necessarily equal future atrophy in that spot. That place may end up with atrophy, but the fasciculations are not the cause. I've also had atrophy where I didn't have fasciculation that I was aware of. That's why its the weakness that matters, not the twitching. The type of twitching, and if there even is twitching needs to be taken out of the equation completely, if one is trying to evaluate the seriousness of what is happening.
As for clinical weakness versus subjective/perceived weakness; in the bulbar area, other than my voice, the growing weakness was so gradual that the professionals pointed it out (or found it through testing) before I noticed it. After they'd point it out, I'd realize they were right, but the weakness was noticed by them first.... Even with my voice, it was others, my family or friends who knew my voice well, that heard the change in it before I could.
But then, in my arms and legs, I noticed it before it could be measured by them. In September the PT at the clinic, and my neuro found weakness in my left hand and my feet, but they said before they would ever be able to detect it, I would have noticed it first. That statement was true of my limbs, but not really for the rest of me.
I agree with Caroline, that the weakness, and prospect of the loss of independence, is what has been hard for me to deal with.
The fact that its usually gradual may be why some are more advanced (like Judylyne's husband) before the person feels something is wrong, and it takes something unusual, like a muscle moving on its own, for them to take notice. Imagine, for an example, jumping into the pool on a hot summer day. The water feels cold when we first go in, because it was a sudden change of temperature, but after being in there for a while, it doesn't feel cold at all (unless it really is LOL)