¿Why E+ recommend use ground temperatures between 15-25 ºC?

edit

Hi all! I am simulating a small house, and if I use Slab pre-processor to calculate ground temperatures, the program calculates ground temperatures, in winter, below 15 ºC. Then if I use those in the object Site:GroundTemperatures:BuildingSurface, EP reports the warming about the use of temperatures below the range 15-25ºC. I dont find the reason in I-O reference. Thank you in advance!