[HarryT]

To elaborate, the Kindle won't fully justify a line if justification would involve inserting too much white space into the line. Instead, the line is displayed left-justified. This will typically happen with a large font size and a line containing long words. This is what the Kindle's done for a very long time (at least as far back as the Kindle Keyboard). It's not a bug, but a deliberate design decision: lines with excessive amounts of empty space within them look a lot worse than left-justifying the line.