As Susan says, if a paragraph appears justified without any extraneous spacing between words, it's because that paragraph just worked out that way (for that screen size and font-size and margin settings). Luck of the draw basically.
EDIT: Actually, on reading your post again. I'm almost certain the "flush left a line in the middle of a justified paragraph to prevent large spaces" that you mentioned is just the technique the Kindle Reader uses in place of hyphenation. It's some sort of algorithm the reader uses that determines when too much spacing would be required to justify and so leaves that line "ragged" instead. It's still not something you can control (as a book creator) at all.
Last edited by DiapDealer; 01-13-2013 at 08:14 PM.