Good question.  Generally poorer bone quality in the upper jaw is just
one possibility.  Maybe a shorter fixture was used to avoid the need for
a sinus lift.  Maybe personal preference.
    In any case, a legitimate question.  You should ask your dentist.

Steve

The more anterior (towards the front) was
badly decayed and the doc splinted the two
together to prevent one crown from becoming dislodged.

Joel

How do I know? I spoke to the Creator about this.
JANDREWSPEAK: I spaketh to the Creator

--
Joel34

Yes, you are correct, splinting is done primarily to help in situations like
low bone density, and the upper jaw is nearly always less dense than the
lower.  Another factor to consider is that #13 and 14 are near the left
maxillary sinus area.  If your sinus anatomy caused the surgeon to use
shorter implants, that is another reason to splint.  Another excellent
reason would be if the patient is a bruxer (grinds teeth habitually).  Also
some sort of mouthguard would be in order usually.

I hate to splint crowns, but sometimes its just necessary.  Good for you
getting implants and crowns!  Congratulations.

Dr Abu

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