I'd bet that data is from sensors with low heat dissipation. Unfortuantely, humans produce ~100W of heat each. If you're using a small underground basement as a place to escape lethal heat aboveground, or even a large underground basement with high people/m³, this added heat will steadily increase the basement's wall temperature.

The rate of temperature increase depends on physical properties of the basement walls: using the first equation here[0] as an approximation, set x to 0, set q"_o to 100W*(# people)/(basement surface area). α is the thermal diffusivity of the wall material [1].

[0] https://slideplayer.com/slide/1676039/7/images/9/Semi-Infini... [1] https://en.wikipedia.org/wiki/Thermal_diffusivity