Delta-Wye transformer differential protection during external line to line faults
Transformer differential protection is not comparing corresponding phase currents on each side of the bank, it is a comparison between an intentional combination of phase currents.
The figure below is from a set of my lecture notes on transformer protection.  The currents shown in red are the primary phase currents, those in green are the secondary currents to the relays.
You can see that the top differential relay only receives \$I_a\$ on its right-side input tap, and \$I_{a'}-I_{c'}\$ on its left-side input tap (both divided by their respective CT ratios).  So, external faults do not upset the differential balance.
This phase compensation (\$I_{a'}-I_{c'}\$) is done with delta connected CTs for electromechanical differential relays.  Numerical relays can do this subtraction internally so they can use wye CTs like I show below.

p.s.  I am only showing currents to the top differential.  You can easily fill in those going to the 2nd and 3rd differential elements.