The Greenwich Meridian.
An explanation as to why the Greenwich Meridian has been placed 100 metres east from where it should have been, according to your GPS.
This is a post from John Sager made in sci.geo.satellite-nav, which has a layman's version of what the references at the bottom have in them.
It wasn't a mistake, just a sequence of steps, logical in themselves, but leading to an odd endpoint:
Basically the meridian was established at Greenwich (latitude 51.5 deg N), using the Airy ellipsoid as the reference ellipsoid. This is defined by survey in the UK and is not quite geocentric. The meridian established for satellite navigation (the original Transit system) was based on a geocentric reference frame where the Greenwich Meridian crosses the equator. This latter meridian has continued, with refinements, to become that used for GPS. The non-coincidence of centre points of the Airy ellipsoid and the WGS-84 ellipsoid gives rise to the 100 metrte error observed at Greenwich, although there is (should be) no error at the equator. This is difficult to check because it's in the Atlantic Ocean off the coast of West Africa!
Click here for a reference.
Click here for another reference.