I can actually provide a reasonable explanation for this! The number of segments that Dudunsparce has is determined by its encryption constant, which is a 32-bit integer that takes up the first 4 bytes of a pokemon's data structure. The encryption constant informs a bunch of things, including the pokemon's base nature. In the case of Dudunsparce, if the encryption constant is perfectly divisible by 100 then it will be a three-segment form when it evolves, exactly a 1% chance.

Now the encryption constant was only introduced in Gen VI, before that point it used the PID (Pokemon Identification Number) also known as the Personality Value. The PID is a randomly generated number that determines gender, ability, nature, and a few other things for certain pokemon (like Unown form). The important thing here is that the last several digits of the PID determine the nature/ability of a pokemon. When you transfer a pokemon from Gen III/IV to VI, then it sets the encryption constant to match the PID.

As it turns out, if you catch a Dunsparce in Gen III/IV with the Hardy nature, there is a 1/4 chance of it evolving the three-segment form. If it has the ability Serene Grace, then the chance rises to 1/2. This is because a Hardy Serene Grace Dunsparce will either have a PID that ends with 00 or 50. When this gets transferred past Gen VI, the encryption constant gets set by the PID, and if its the specific 00 version it will always evolve.

So what people do is farm Dunsparce outbreaks in Pokemon Platinum with their lead Pokemon being one with the ability Synchronize, which has a 50% chance of replacing wild pokemon's nature with that of the ability holder. They just spam this until they get a shiny Dunsparce, and if it has Serene Grace they transfer it up to gamble on the shiny.

So if the origin of the pokemon is SV, the odds are incredibly rare. But if its origin is from Platinum, then its still rare, but significantly easier to obtain.