Last summer I bought three digital thermometers, similar to:
You guessed it. Each gave a slightly different reading (two or three degrees difference) when placed at the same time into one glass of hot water. Even gave different readings when placed side by side on the living room carpet.
And just for fun, your car coolant's temperature gauge reading is probably just an "average" of the varying temps of the coolant as it passes by.
ποταμοῖσι τοῖσιν αὐτοῖσιν ἐμβαίνουσιν, ἕτερα καὶ ἕτερα ὕδατα ἐπιρρεῖ.
Or words to that effect ...
OK - back to my cave.
Tom M.