I think most people don't care because it's a random formula. It should have been ((kills + assists) / deaths) but for some reason 343 decided to make it an average spread sort of thing where it's ((kills + (assists/3)) - deaths). Then that number is averaged per game.
It's just strange to me because someone out there decided than an assist, no matter what, is worth 1/3 of a kill. AND also decided to call it KDA just to make it extra confusing. It would have been a lot better to just stick to the usual formula or call it something else.
Halo Data Hive seems to calculate it using the traditional formula though.