The fact is that what we are doing is not what they are doing, nor exactly what they did. At most it is related. How it went over, if it did, nobody knows. It probably went through a few generations. Perhaps someone came home with that shape that they got from somewhere else. Perhaps a child remembered this from a parent’s house. Who knows. Who cares?
We know that it is not the oldest Minhag. Either someone established it and it took off or it slowly spread. Either way, no matter what level of Am Haratzus might have existed, one thing every Jew knew: not to make crosses, on your bread or on your head. So, if it somehow did trickle over through osmosis, it isn’t exactly a copy and was never meant to be. At most it is an influence.
To harp on the technical grandfather of a custom is as silly as finding that Moshe Rabbeinu didn’t wear a Streimel, enlightening. In either case something was adopted and evolved and incorporated into the Jewish people while its meaning followed and became part of it.
Just like a Gadol can create a Minhag or Hanhagah because of a certain invention or meditation, he can just as well add this meditation to an existing Hanhagah.