Couple of thoughts with the caveat that I've flown on this plane a bunch but always in either C+ or row 15 (exit row), I've never sat in row 13:
- Not sure why 14 is considered preferred, it's in front of row 15 (exit row) so no recline
- Typically the first row of Y after C+ has less legroom which may be why row 13 is designated as main instead of preferred