Annie is right,

Many times physicians look for one unifying explanation for everything that is going on with the patient (Ocam's razor). This is more "elegant" and creates a feeling of a complicated puzzle put together.

But, more times than not, there is, in fact, more than one thing going on (Saint's triad). This can create a very confusing picture and lead to wrong directions.
http://www.nejm.org/doi/full/10.1056/NEJMcps031794

Also, quite often physicians lose their simple common sense, which gets buried under an enormous pile of tests, documents, guidelines and boxes they have to tick. Many times they are just too busy with all the documents and charts that they just don't have time to listen to their patients.

The hallmark of myasthenia is that there is disproportionate decrease in muscle strength with activity. You can blow one time with near normal force, but the second time it will be half of that. There is also at least some improvement with rest. In some patients the weakness at rest is more pronounced and in others less.

Possibly, you could have myasthenia with mild Addisson's disease (which can also lead to muscle weakness) and this combination may create a very confusing clinical picture or appear as something else.