Differential Protection of a Transformer

It is true only in case of Star-Delta or Delta-Star Transformer. It is done to compensate for the 30 degrees phase shift in such transformers, so that the phase shift is not seen as a fault by the relay.

This kind of reverse connection is actually not required in modern microprocessor based relays.This phase angle difference can be compensated in the relay itself.Means no need to connect CTs in star mode in delta side & vice versa.

These change of CT connections are not required for the modern microprocessor based relays of Siemens Siprotec relays and Areva MICOM P632 relays.Similarly for ABB and Schneider make relays.

as electricalexpert65 said, it is to compensate the currents to make this differential or operate current zero.

if you really want to know it, you have to draw the phasor diagram of this currents, from the primary phasors down to secondary phasor from CT connections. you can experiment on this through solving it. And then, you will know the answer by calculating the differential current or the operate current.