Have your kids been over there much? 

Had the same problem with my father, about the same distance. He was always getting a substantial amount more ( or less) rain than me. And I knew he was not exaggerating. After this went on for several months, I finally figured it out. One of my boys was adding water or taking it out to his gauge after every rain ( his sister ratted him out). 

Adding, or taking rain out of Grampa's gauge is no bueno!

I had to sit down and have a talk with that boy....