I've seen this on other readers including the Kindle. As JSWolf said it is to prevent too much space between words. On the Kindle it is even more noticeable as you will periodically see a large gap at the end of a line when all the other lines are full right. I assume they use an algorithm that decides whether to justify or leave a gap depending on how much space there will be between words. I certainly prefer hyphenation, but I've also seen readers that overdo that as well.