I farmed Ramboy for 30 runs on normal, and got 1 fortune box, 25 mix boxes, and 4 runs with nothing.
So from my testing, it seems that normal for RB has almost a guaranteed chance of giving you a mix box, but has a much lower chance compared to insane RB for a fortune box.
I did only 15 runs on insane RB last time, and got 3 fortunes, but not as near as many mixes.
The only other variable was my rank. This time, I was rank 13 instead of 12, but I doubt 1 rank would make this much of a difference.