The titanium grid fins are also larger, which gives them better control authority. The center booster (like regular Falcon 9s) carries the interstage. This is

The titanium grid fins are also larger, which gives them better control authority. The center booster (like regular Falcon 9s) carries the interstage. This is light weight, but has significant aerodynamic effect when re-entering engines first for landing, like the feathers on an arrow. The side boosters lack this - the nose coses are much smaller - so are less stable aerodynamically. The likely reason for using the titanium fins is to give better control. They don't need the extra heat resistance for the side boosters - the center booster will have a toastier reentry as it's going much faster when it shuts down.