Forza Horizon 2 Perforance Index Question

So, in Forza 4 and Forza Horizon the PI was like this: F, E, D, C, B, A, S, R3, R2, R1, X (FH4 Only)

Yet I have seen in Forza Horizon play throughs a new rank, S2, at 998, the max rating for R1 in Forza 4 before it becomes an X class.

So, what are the differences?

See this: