Why did she use "one day" instead of "some day"? Is there any difference in nuance?

Rebecca

He used "one day" because "one day" is often used to refer to something probable happening in the future, while "someday" refers to something more uncertain happening in the future. That being said, "one day" and "someday" are fairly interchangeable. Ex: One day, I plan on owning a dog. Ex: I hope I have children someday.