Hi! I’m going to post this publicly because personally I find it an interesting topic. For something as short lived as a betta, a year or two can be a big difference in life span. The reason for this, to my knowledge and based on what I’ve generally learned with my degree, is two-fold.
Genetics plays a huge role. A well-bred betta is going to be less prone to things like swim bladder disease and will be overall healthier, better able to fight off infection with a better immune system. Bettas with shorter fins will have less strain on their bodies and this too may play a part in length of life. And too, as we tend to see in some other animals, bettas from stock that tend to be long lived are also very likely to be long lived.
The second factor is largely going to be quality of life. Obviously a betta kept in a small cold bowl isn’t going to life as long as a well cared for betta, but what is also important is early life care and that comes down to the breeders. Like all animals, suffering in early life will severely affect development that will be seen throughout an animal’s whole life. Reptiles who don’t get proper calcium and lighting can have terrible bone deformities. Dogs raised alone tend to be more neurotic (basically they lack some ability to understand dog language). Human babies in orphanages who do not get held and coddled have physical growth problems.
I’d probably place money that a lot of our pet store bettas are bred in mass production systems that might have prevented some of the ideal care.
Shipping methods and other stressors throughout life may also shorten lifespans, and diet can play a role, too.
And then of course, you know, some bettas just are different. And it can also be hard to tell how old your betta is in the first place. Unless you happen to ask somebody who knows, you might not have any idea how long your betta has been sitting on the store shelf. At work there are bettas who have been on that shelf since I started working there 6+ months ago.
Short answer- it’s complicated!~ I would definitely welcome any input from others! (: