The answer has to do with the head-shapes of these two people.
R1b is in the Pelasgians and the Ligurians, both of whom are long-headed.
R1a is round-headed as Central Asia seems to have always been.
Language can be learned... and certainly was, as long ago as Mitanni overlords taking Hurrian names.
The whole answer almost certainly has to rest upon this three-legged stool.
... I found an answer on quora who tried to pretend to himself that the Germans and Sarmatians were indigenous to Europe.

Who's who may be the most important question of our entire existence, given the great lengths people will go to try to change history.