Slightly more than idle speculation:

There was a goal to unify numbers; this couldn't be done when openSUSE
Leap was announced since they were already on 13 and SLE was about to
release 12, and really it was openSUSE making the big change back then.

13 was already used by openSUSE (like on my laptop right now, because I"m
so happy with it I keep resisting moving just in case something newer
isn't as great.... silly, I know).

13 is unlucky in western cultures, and 14 in eastern cultures. Why not: 15.

Good luck.

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