One possibility is that we are talking about two or more different diseases. Another is that there is some variable involved.

My own view- While the sense of smell is touted as the first symptom, that is not quite true. It depends on if you mean the first one noticed by the patient or the first one that can be detected by scientists.

If the latter, there seems to be a tie of sorts between the olfactory bulb and the nerves found in the wall of the stomach (the myenteric plexus). These are the first places that German researcher Braak found Lewy bodies appearing.

These two areas share a common trait - they are in intimate contact with the outside world. We breathe in things that stick to our sinuses. Most are swept out, a few penetrate the defenses in that area, and the rest end up in the stomach. Something in the environment. It is airborne (else the olfactory bulb would not be affected). It is probably a pollutant that came with industry.

It is also probably a "secondary" factor in that it spells trouble only for those already predisposed (else more would be affected). And it should be capable of moving along the axons to match Braak's other observations. This means it has to be tiny particles or even molecules.

Although researchers haven't caught on yet, there is an ideal candidate available. Soot. Or, if you prefer, Ultra-Fine Particulates. Produced by burning diesel, coal, charcoal, etc. these tiny specks of matter can penetrate deeply and pass through most barriers. They are also capable of acting as ferries and carrying toxins with them. If inhaled by a person who already has an inflammation-prone system, the resulting inflammation allows even deeper penetration while also revving up the host's immune response.