The answer, quite simply put is the property has progressively changed over time.  Grass and its’ seasonal dormant root cycle along with clippings from mowing and pollutants have substantially raised the site grades while humus matter in beds at the foundation remain lower.  This creates a negative grade to the structure.  Watering programs that sustain landscape encourage a controlled percolation depth (hard pan to the farmers) which then causes sub-surface waters’ lateral movement to the negative grade at the structure.