Considering the amount of attack rolls that this game must make a 1 in 400 chance will happen, and happen often; you were just on the wrong end of the RNG goods favorites list
It's not just 1/400 attack rolls, it's 1/400 attack rolls with advantage. That's when the game should be computing two 'd20 rolls' and taking the higher, and yet getting critical misses with advantage is still much more common than should be possible if their RNG system was using anything similar to actual probability as a basis.
