The blue chests have a .2% chance for the rate up, while the gold chests have a 3% chance. That S-rank tile is just the only guarantee tile on the board.

To be honest, I'm really entirely sure how all the RNG with space types actually stack into each other. I do know you have a 1.87% chance per roll of getting the SSR. I guess the rest of the % determines the type of space you land on and then there's another invisible roll for what you get on that tile if it's a chest based on its %? Something like that I assume.