In-rush current phenomena happens at the time of SWITCH -ON in all the 3 phases as such the difference in their values in all the 3 phases are not signifiant as it is dependant upon the Air core reactance and the residual magnetisim in the core at the time of charging the transformer .Further , the inrush current persistes for a longer time and it's value diminishes with time .
Diffrential protectin is provided between lines or between HV and LV . A internal fault in a phase resultrs difefrence in currents in 3 phaes can generate diffrential current for operation of relay and this does not reduce with time . ( Internal faut in all the 3 phass do not hapen inside transformer )
Transformer inrush current contains lots of harmonics especially the 2nd Harmonics. Differential relays use the 2nd order harmonic to block or offer a restrain for operation.
check this paper out for a detailed explanation: https://cdn.selinc.com/assets/Literature/Publications/Technical%20Papers/6615_LowSecondHarmonic_BK_20140210_Web.pdf?v=20160224-200704
The key issue is Internal fault and Inrush current . An occrence of nrush current means the transformer has gone through the situatin and under work afterwards.
An iternal faut implies the trnasformer has stopped working after the fault - in this case no further analysis is required/warranted by the user since th etrnaformer has failed .Blocking by relay or operation of relay are only helpful if the trnasformer is working - itherwise the data is of no use for any practical purpose