Just a thought ...
One reason for apparent differences in "survival" (hate that word ... much prefer "living") after diagnosed may be that patients are diagnosed at different stages of the disease. Looking back, I know there were emotional/behavioral changes that could have been the start of FTD (and ALS) 10 years ago. I also had baffling physical episodes that were never explained 2 and 4 years before the start of symptoms that I paid any attention to. I date the start of ALS with when my slurred speech started, but I'm pretty sure there were things going on before that that I didn't consider "symptoms" of anything ... such as long, uncontrolable bouts of extreme yawning, etc., which were very embarrasing at work!
Also, with cancers there are slow progression types and fast progression types, so I imagine it is the same with MNDs, since there is such a large variation in how we all present. There must be reasons for the variations, and understanding them will come from research.