Finding peak rpm can be complicated with a constant speed prop. Better check the POH for the correct leaning procedure
Edit: I just checked some sources and found this
quote from John Deakin
But I’ve flown a couple of 182s that are simply hopeless, and I’m tempted just to leave the mixture full rich and forget it. Pity, the Skylane is a marvelous airplane, but that TCM O-470 engine has the worst mixture distribution of any powerplant on the planet.
I don't have any experience with that engine myself, but to elaborate on the previous post, you might see different effects based on where your mixture is, what the outside air is doing, whether you may have had a touch of icing perhaps... And if John's quote is correct you may see different effects based on the mixture distribution at that time. If three out of the six see a better mixture thanks to your carb heat application, that may lead to a higher MP but it could be the other way around as well.