I believe there are many other factors as well...
One I recall right off the bat - there are standards for construction of a station, including what to paint it with.
The standard was oil-based paint, and the standard was changed at some point to latex paint. Turns out that change has a heat effect change as well... and, naturally, in the direction you’d expect if you wanted your stations to report higher temperatures over time. The phase-in of the paint change was gradual, as stations needed new coats of paint, or new stations get worked into the system, thus the overall increase in temperature was moderated (easier to hide, and portray as the glowbull warming they wanted).
I’m surprised that a fan is not built into the system, so that all you get is temperature, not trapped air heat/cold.