Yes, it is. Do you think the AC comes and goes that symmetrically? Seems slightly hard to believe but maybe....
Our AC operates on a "Bang-Bang Servo" concept, where the AC comes ON until a lower set temperature is achieved, then OFF until a higher set temperature occurs, then the cycle repeats. The AC period is shorter during the day, but longer at night when the outside is cooler.
Copper has a very high TC and thus becomes a temperature sensor rather than a fixed stable resistor. You can cover the entire measurement with a blanket or thermal insulating box, this will reduce the measurement excursions.
If you have another plotting meter, get a thermistor and plot the temperature along with the measurement, they should correlate well.
Best,