This is the exact answer. The thermometers aren't measuring the world. They're measuring a tiny piece nearest to civil development, and that civil development is able to shift the local temperature. Ever notice inner city areas are hotter than even a little outside of city limits? So do the weather stations. 

 Yes, also natural urbanization over time. 

A measuring station that used to be on the outskirts after decades of growth is well within the concrete/asphalt "heat island". 

 Precisely. Those measuring stations are usually not too inconveniently far from urban areas, airports being a logical place, which are not only not immune to development but actually invite it.