I have no idea what you're saying there.
AFAIK it does work that way, two equal masses of different temperatures, mix them together the end temperature will be the average of the two.
Yeah, your gravel, the tank itself etc will change the end results, but I think that would be drowned out(Pun intended :lol: ) by the couple hundred kilo masses of the water.
Which is why I say my temp would drop to 16ish when I do a 50% water change, over winter my water comes out of the tap...Not freezing, but close enough if you were to jump into my rainwater tank. I measured it once at 8°.