Answer first, context after
Why does my thermostat read a different temperature than the room feels?
Usually because the thermostat is telling the truth about the wrong spot. A thermostat in a sunbeam, above a supply register, on an exterior wall, or in a stuffy hallway reports its own microclimate, and the whole system obeys that report. Location explains most of these mysteries; a failing sensor explains a few; and sometimes both devices are right and the house's airflow is the liar.
The thermostat is the only vote that counts in your HVAC system, a single sensor in a single spot commanding the whole house, so when its reading and your comfort disagree, the entire system runs on bad information. The good news: the usual culprits are cheap to identify.
The location problems, in order of frequency: direct sun, even an hour of afternoon sun crossing the wall, convinces the thermostat the house is warm and shuts cooling down early. A supply register above or beside it bathes the sensor in conditioned air, so it declares victory the moment the system starts, the classic cause of short, useless cycles. An exterior wall lets outdoor temperature bleed into the reading through the drywall. And a dead-air hallway, the most popular thermostat home in America, often runs warmer or cooler than the rooms people actually occupy, which means the system faithfully conditions a hallway while the bedrooms file complaints.
The honest experiment before anyone bills you: put a cheap thermometer beside the thermostat for a day, then in the room that feels wrong. If thermostat and thermometer agree with each other but not with the far room, the thermostat is innocent and the airflow to that room is the suspect. If they disagree in the same spot, you have a sensor or location problem, and the sun-and-register checklist above usually names it.
The aging-sensor case: thermostats drift, batteries fade, and a decades-old unit can read a couple of degrees off out of pure age. Some digital models offer a calibration offset that papers over small drift honestly. A thermostat old enough to drift is usually old enough that a smart replacement, $240 to $420 installed, fixes the reading and adds the scheduling that does the real bill-cutting.
When relocation is the real fix: moving a thermostat off a sun wall or away from a register is modest, unglamorous work, frequently bundled into another visit, and it beats any amount of arguing with the settings. The system can only be as smart as the spot it listens from; give it a representative one, and half these mysteries close themselves.
Still stuck? That's what the truck is for.
The diagnostic is $89, waived when you book the repair, with a written flat price before any work starts.