Well, everyone's math on probability looks correct. (Part of my job is doing statistics) However, the elephant in the room here is the computer -- they aren't really random. I think we've all seen that were the RND seems to "clump" or "chain" as others have described.
The reality is " surprising as it may seem, it is difficult to get a computer to do something by chance. A computer follows its instructions blindly and is therefore completely predictable."
Thus the "random" in any computer game is really pseudo-random, and if you know the seed and algorithm it is completely predictable. See https://www.random.org/randomness/ if you want to know more. If you want real random then you play a game like Warhammer where you roll real dice.
I guess I know behind the scenes thats how it works and just live with it.