The actual explanation is very involved, but it boils down to the fact that due to gravitational anomalies in the area around Greenwich, the "straight up" verti

The actual explanation is very involved, but it boils down to the fact that due to gravitational anomalies in the area around Greenwich, the "straight up" vertical that was used to establish the Greenwich meridian as the origin for universal time does not pass through the center of mass of the earth. The coordinate system used by GPS (WGS84) has the center of mass of the earth as it's origin. The plane that defines the reference meridian (the 0th meridian that is) in that system has to be parallel to the one used to establish universal time, in order to maintain UT consistency. So, when you extend the plane of the WGS84 coordinate system from the center of mass of the earth, parallel to the vertical used at Greenwich, which, again, does not point exactly at the center of the earth, the plane cuts the earth's surface 102 meters East of the original 0th meridian.