This is the kind of thing that depends on the author and the writing. A lot of times, an author will use various names for various series that are in different genres. For example, if Mary Sue Genoese writes a fantasy series about elves, and simultaneously wants to write a science fiction series about black holes, she might want to use a pseudonym so people in the sf/f section won't pick up her book about black holes thinking it's about elves.

Sometimes an author well known for sf/f or historicals will want to write, oh, a mystery. Said author might not want that mystery to show up on the sales track, because it would affect the sales of the sf/f books or whatever. Then the author would use a closed pseud -- either the author wouldn't even tell the editor, or the editor wouldn't tell marketing/sales. Etc.

A lot of times authors will make public these pseuds, so their fans who read cross-genre can pick up their books.