The answer is very simple.

This test is much less sensitive then what it is assumed to be.

The reasons for that are not important. It could be because of a different underlying mechanism, influence of environmental factors, gender differences. The bottom line is that there are patients with maysthenia who have a completely normal test.

The very high sensitivity of nearly 100%, is based on studied with significant methodological problems.

I do not think this was done deliberately. This test seemed at the time, to solve a serious problem. It provided an objective way to diagnose an illness, that was diagnosed clinically before.

It also provided an explanation and better understanding of the mechanism of this illness.

I do think it is time to correct this error. Some neurologists understand the true limitations of this test, and some still do not.

some still think that a normal SFEMG done in a weak muscle rules out MG as the cause of this weakness even if there is no reasonable alternative explanation. Just like the top leading expert described in the book written by Chloe Atkins.