White type and black type have different visual weights. So the "Unbound" ends up looking thinner than "Mortis", even though they're actually the same size. Most likely part of the point of making the title two different colors was to keep either word from being devoured by the background, since the white "Mortis" is on a darker section of the background than "Unbound". The apparent difference is just enough to snag the eye, but not enough to look totally "wrong" or look like an intentional size difference.

Hopefully that's helpful and not desperately nitpicky sounding. :)