It looks as thought the Kindle has a limit of two consecutive spaces between words when full justification is set. This is a halfway acceptable compromise to allowing any number of consecutive spaces to ensure that there is full justification or else!
I'm in the group that prefers left-only justification, but the current system isn’t as bad as it was for a while when the Kindle would allow as many spaces as it took to get fully-justified.
This behavior, while specified by the publisher, is not cast in stone. The Stanza app on the iPad will allow over-riding those settings and gives control over left/full justification, inter-line spacing, inter-paragraph spacing, paragraph indent spacing, and full margin control. And Stanza also has the ability to turn hyphenation on and off.
I mention all that not to show how wonderful Stanza is, but to show that it's possible. The question is: Why does Amazon think we don't need these things?
Mike