The reason of ‘their’

The reason of ‘their’

The passage below comes from Women, Race & Class by Angela Y. Davis.

He cites the story of a woman whose great-grandmother always described with enthusiasm the battles which had earned her the considerable scars on her body. But there was one scar she persistently refused to explain, saying, whenever she was asked about it, “White men are as low as dogs, child, stay away from them.” After her death, the mystery was finally solved:

She received that scar at the hands of her master’s youngest son, a boy of about eighteen years at the time she conceived their child, my grandmother Ellen.

Let me paraphrase the last sentence first.

The man who gave the woman’s great-grandmother the scar was her master’s youngest son. The brutish son was about 18 at the time she conceived their child, the woman’s grandmother.

(Do you agree with my rewording?)

Now, a question is in order.

Why the underlined ‘their’ instead of his, that is her master’s youngest son.

Is it because there is no way of pinpointing whose son the child is? Does ‘their child’ imply that she could be the daughter of either the master or the son?

Thanks in advance.