Probably because the motor is high compression and tends to run on the hotter side given the occasional oil temp issues.
Hotter plugs are probably prone to some pinging. Although, if you run 93 AKI (or higher) you would probably be fine with slightly hotter plugs -- remember, the engine is tuned and set up to run on 91.
That said, read p. 9-4 of the owner's manual... they claim some light knock under load acellerating up hills is normal
You could also try playing with the gap, I suppose, but if it runs good, why mess with it?