The short answer is: there was a historical shift between the first global datum and the Greenwich meridian; and it continues to move (slowly) because of continental drift. You can find more detailed information on this site
In the late 1950s (under the auspices of the US Navy), the Applied Physics Laboratory (APL) of the Johns Hopkins University began the development of what was to become the world's first operational satellite navigation system. Known as Transit, it worked by making use of the Doppler effect, the same effect that makes a siren carried by a moving vehicle change in pitch as it passes. The surveyed longitude of the Laboratory's site in Maryland, as measured in the North American Datum (NAD27), became its assumed longitude in the first World Datum, the APL datum. It was this pragmatic adoption of the longitude coordinate on one ellipsoid as the assumed value on another that has caused the apparent shift not only in the position of the Meridian, but also of all other locations.
The size of the shift remained unknown until the summer of 1969, when an opportunity arose to measure it. A satellite receiver was set up on a platform above the roof over the Airy Transit Circle at Greenwich. The results showed that fixes resulting from the use of the satellite navigation system should have their longitude values shifted by 5.64" if the Greenwich (Geodetic) Meridian was to have its longitude as zero in this system. Although an academic paper on this subject was published in 1971, it appears to have been largely forgotten about until the mid noughties. The offset (since refined) also applies to the WGS84 datum used by current GPS systems. WGS84 was adopted as the global standard for air navigation on 1 January 1998 and soon afterwards by hydrographers for use on electronic and nautical charts.