Secondarily, it was made to be smoother, more Greek, and possibly more Christian and theologically exact. These recensions cannot, however, be identified. This seems to have been the last great revision of the Septuagint. The nature of the difference between LXX and MT varies from book to book. In Isaiah, it may simply be the incompetence of the original translator.
Lesser differences occur everywhere.
At this point we need to step back a little and examine the situation at a higher level of abstraction. One course is to treat the MT as the basic text, preferring it at all points where it can be construed. The LXX is used only where the MT is corrupt. The logical fallacy with this is that makes no sense. If the LXX has value at all, it has value everywhere.