Why does Wolverine's PI reduce when he is Ranked Up?

I had this surprising incident with my 4-star Wolverine. I had reached Rank 2 with PI of 1853. When I ranked him up to R3, I was shocked to see his PI go down to 1820-something. Same happened when I ranked him up to R4. Before the rank-up, he was 2570 and then reduced to 2540-something.
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