The answer is that atypical presentations are typical for about 5% of patients with any given disease. (roughly). Also, many times such patients are more likely to have normal tests (after all those tests were found by checking typical patients, so it is not surprising).
Atypical symptoms with normal test results make the given disease much less likely, but much less likely is not impossible.
(after all someone does win the lottery each time).
When you have such an illness, you need a smart, open-minded, caring and dedicated physician who will be ready to be there with you, through the very hard diagnostic process. Someone who will be ready to treat you, even if your illness does not yet have a name.
This is a citation from an e-mail discussion I had with one such physician:
" even if the diagnosis is not absolutely clearcut at a molecular/biochemical/ structural level it is important to treat what is before your eyes and manage the consequences of a process even though cause not known. And it is the combination of that and being open minded enough to continue to explore the diagnosis as more information comes to light that makes all the difference."