Curious case of SBI and PSU Index

For the last few years PSU bank index moved in tandem with SBI.

SBI raises - PSU Bank Index raises
SBI Falls - PSU Bank Index falls.

While PSU bank Index fallen 500 points (14%) from March 2019
SBI has just fallen 20 points (6%) from March 2019

Even though BankNifty is beaten down SBI is standing tall. Expecting a good move in SBI after election.