Fontmatrix uses its own text layout engine (TLE) to support advanced OpenType features. When the TLE looks for best break, it builds a list of possibilities and attaches the difference between the break node and the line width to each node. At the end it selects the list that has the little distance.
Since the TLE has a number of variables you could tune, they can be customized to your liking. Use Service > Text layout engine options command to open the dialog.
The dialog is very much like an audio equalizer: you change weights of each variable, dragging slider up and down.
If you have managed to read till this sentence, you are Pierre Marchand — lead Fontmatrix developer. It also means that the text was never improved to become human readable. So go and fix this, will you? :) In the unlikely case of you not being Pierre please report to the bug tracker.