Spark Plugs & AFR changes

[RICE RACING]

So I have been doing this shit for like 1000 years (or so it seems) and I thought I would post this up as no one has ever bother to do so in the internet forum shit hole world that is non ebay porn related postings.

So here goes:
Who has ever noticed that their full load AFR changes, when you put in a different spark plug (say hotter)?
* No audible misfire, no knock.
* No loss of power.
* Nothing to alert you to say cooler plug not being fully functioning.

I did a test today where when swapping out one set of spark plugs of cooler heat range to a hotter type found an air fuel mixture that normally data logs at ~11.5 to 11.7 range leaner to 11.9 to 12.1 range. Absolutely nothing else changed (same boost, rpm, type of load test, exactly same power and performance measure) just swap out spark plugs, that's it.

Suppose it is not something people are too exposed to unless you have a car fully instrumented as I do or are not testing it every week as well, but wonder what other cunts have seen found over the years, when running legitimate back to back tests