Muscle fatigability and fluctuating weakness is the same thing.

Think about this way.
When I was a kid my grandfather had a small cinema. He would sometimes let me look at movies frame by frame by rolling them slowly or we could move them very fast.

Think about a charlie chaplin movie where you see (in normal speed) how the cake flies over the room and sticks to someone's face. In slow motion you will see the cake slowly moving over the room. In very fast motion it seems to just appear on the other person's face.

Muscles don't just abruptly stop. The question is how fast those changes occur.

In patients with "classical" MG this process is slow and gradual and can easily be detected by someone looking at it.

In more "crazy" variants of this illness, it is more like the charlie chaplin movie I described.
I know that for me, fatigability is not linear. At first it is very slow and delayed, but at some point it becomes very rapid.

You can compare it to a charlie chaplin movie in which you initially move the film frame by frame, so slowly that you can hardly detect any difference in the place of the plate and then when it becomes totally boring and everyone loses interest you start moving it so fast that it seems the plate just got stuck on the person's face from the middle of no-where.

That is why almost every neurologist who examined me thought I had a completely normal neurological examination and could not understand why I was unable to move, talk or breath after it. Or one neurologist who had to score my QMG was very puzzled by the fact that I had no ptosis when she asked me to look up, but my eye was nearly closed at the end of her examination. She couldn't decide if she should score it as 0 (no ptosis) or 3 (spontaneous ptosis) and she shared her dillema with me. (that is how I know about it).

I hope this clarifies it a bit.