During the climb at a constant CAS, the TAS is increasing, thus, a portion of your excess thrust is used to ACCELERATE the aircraft.
At Cross-over, Mach is constant and CAS and TAS are decreasing, thus (1) the excess kinetic energy of the aircraft is translated into climb performance as the aircraft DECELERATES, and (2) the portion of the excess thrust previously used to accelerate the aircraft is now available for improved climb performance.
Regards,
Old Smokey