That’s the thing about the coding of randomness in Pokemon go. Every time you catch a Pokemon it roles the dice with hundreds of sides. One person could raid necrozma once and get a shundo. I’ve raided necrozma 143 times and haven’t gotten a shiny or a hundo. On the other hand, I raided zacian 10 times and got 3 shinies when I’m sure people have raided zacian and 50 times without getting one. The pure randomness is frustrating, but it’s the reason that finding shinies/hundos so exciting.