It's RNG like everything in this game. Unfortunately we like many raiders have just been unlucky and gotten the same thing over and over. Look at all the 29's who've raided every week since launch and still haven't gotten the gear. Yes your maths is correct and the chances of getting it 4x in a row is next to none. But the chance is there. It is mathematically possible that we could get it every week for the next 5 years. Hopefully and probably not though lol.