Why is left justification style not filling text frame on RHS, where as justified with last line left does?

This is a paragraph style with left justification that leaves a lot of white space on the right hand side of the screen.

When the paragraph is changed to "Justification with Last Line Left Justified" it puts several more words into each line and fills the frame with text:

Any obvious reason why this is happening?

I can post a file if need be.