I noticed this in SM quick reference chart today. Timing is different in 4WAS models.

Ignition timing
(BTDC at idle speed)
15° ± 5° (without 4WAS)
11° ± 5° (with 4WAS)

Any ideas why this is the case?

 

I can't offer any insight into this particular issue, but I can comment on Nissan/Infiniti's tendencies in the past. On the first gen cars, the automatics would do some weird timing things between shifts. Some cars saw 20 degrees of timing being pulled between shifts, while others saw >25 degrees.

I know that doesn't have any bearing on what you've noticed, but I still find that fact interesting.