idle rpm increase with temp

[ollykiwi3]

just a general question regarding engine temp and spark plugs

i was doing some playing with idle tune the other day and have gone from 25deg adv with throttle blade almost closed, to 16deg adv with throttle slightly cracked to try and stabilise a hunting idle after start up. been bugging me for couple years now so just wanted to try another approach.
anyway, while idling the engine temp rose to just over 200degF, and about the same time, the engine speed increased and sounded cleaner. i looked at the AFR and it was around 12-12.2, when it would normally sit on 13.5.

this was strange and its the first time its happened. so it got me thinking, : is it possible that the heat range of the plugs (ngk6) is too cold for normal operation, and works better with a hotter engine. Therefore should i change to a hotter plug(ngk5). is this why i saw an increase in engine speed??
AFR165 heads, with 10;1 comp, where AFR recommend using 1 number cooler plugs from standard, which is why im using 6 series.

if i manually try and run 12.2 AFR at idle it doesnt like it, so i dont think thats the answer, but i still dont know why it ran 12.2 anyway. i will have to try re running it this weekend and see if it does it again. will log it next time and post if it re appears.

just asking if its a possibility in theory right now.

thanks