That's a Lot of Dog Years

I am a dog lover and have two of my own.  Today then, the news item about the death of the oldest dog in the world caught my attention.  Chanel, a female wire-haired dachshund that was 21 years old (that's 147 in dog years) died of natural causes in New York.  Now my question is how do they know she was the oldest.  Not to take anything away from Chanel (21 years is a very long life for a dog), but I would not expect canine ages (especially for all of the mutts of the world) to be meticulously documented in any type of globally searchable database.  Apparently they might not really know, as now the owner of a dog named Max from New Iberia, La., is claiming that he marked his 26th birthday on Aug. 9.  Is that 182 in dog years?  If large breeds, on average, live shorter lives than smaller breeds, is the concept of a universal "dog year" equivalent really the same for all types of dogs?  And finally, even at age 21, I have never understood the saying that a person or animal died of natural causes".  Sure, it is natural that every living being eventually dies.  But something (e.g., heart failure, etc.) always triggers the event.  That event, and not "natural causes" is what caused the death.  What do I know though?  I'm just a Blind Bambi.