Why use a pseudonym? The reason is very simple. Ian Bullock writes different kinds of fiction, for different publishers, and the convention in publishing has long been that unless you're a celebrity you can't do that under the same authorial name. Readers would be unhappy to pick up a new book and find they were looking at a work of romantic fiction or children's literature instead of the thriller they believed they were buying, or borrowing from the library. The promotion and marketing departments of publishing companies would be unhappy, too.
"Latus" is the family name of Ian's mother. So: Dan Latus.