The given ambient temp is based on the location of the thermometer's location. Where I live, the official local temperature is based on the thermometer located at the airport. But that doesn't mean that ambient temps at other locations around the city aren't higher or lower. In fact, the temperature in the heart of the urban core is probably hotter due to the "heat island" of the buildings and streets.
If I have a thermometer on my patio that is facing south and the sun has been blasting the concrete for a couple of hours...and it reads 90° F...if I go stand under the big oak tree in the back yard and take a thermometer with me...more than likely it will not read the same 90°. Does that make the temp taken on the patio wrong? (NO). Or if I walk around to the north side of the house and take the ambient temp from the porch which has also been in the shade all day...the temp will probably not (again) read 90° F, either.
If your car's ambient temp sensor is located underneath the car near a concrete or asphalt parking lot...then it is more than likely to read higher than the temp reading from a thermometer not located that close to the ground and radiant heat.
If it is 90° F outside and you come back to your car that has been parked in the sun after 8 hours of work...do you expect the interior of your car to be 90° F? No...but you've adjusted mentally that the interior has been affected by radiant heat...so give your car's ambient temp sensor the same benefit of the doubt. It, too is likely to be several degrees off from some anonymous local temp reading...it is reading the temp from where it is located.
