Why there is not a sqrt(3) factor between rated currents on the delta-wye transformer?
The square root of 3 factor has to do with the voltage relation between the 3 phases and neutral, which normally is consistent. That they are 120 degrees out of phase is a moot point. Current depends on the load and is not always a balanced load.

Since the load is unknown, dynamic and maybe unbalanced it cannot be used as a reference for static equations such as the square root of 3.

This assumes all 3 phases are sampled at the same time, so there is no artificial add-on delay in current readings.