It's hard to say. When you do family history research, you find lots of variant spellings. In times when many people were illiterate, they might not know the "correct" spelling, and it would be up to the registrar to make an intelligent guess! Therefore, my "Lawrie" ancestors appear variously as Lawrie, Laurie, Lowrie, and Laur. The same may perhaps be true of Boyd and Boyle.