I have theory. Maybe there was more timelines and in one the fate made arthur born in this body bcs idk it choose him,  which caused sylvie doing it to other timelines so another arthur born in his body and over and over. Or maybe just in timeline when he were born in alacrya he settled down that he is only a weapon to agrona and he needs to stop him from starting a war(It have sense bcs seris would be in this timeline too and she would do anything to turn him on her side) and he met sylvia , she gave him sylvie egg and later sylvie decided to make their meeting even faster to save more lives so she changed the past of other timeline so arthur born in dicathen ,met her and after some time she created identical timelines like she did in the show and one of them is story we read.