This is temperature of air (big amplitude) vs temperature of ground X meters undergrounds. If you go deep enough it doesn't matter how hot it is outside - the temperature is basically constant the whole year.
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].
http://www.rynekinstalacyjny.pl/images/photos/24/3877/__b_An...