good question. i think they just apply the latest (at the time of release) patches and include extra maps that were in packages or maybe maps that were created in-house. im not too sure what else is included with the RTCW GOTY version but that was generally what happened in the past.
actually, after doing a google, looks like there was a GOTY map pack that was available for download for those that had the original version...or in the case of those users whom bought RTCW as a GOTY version, the map pack is already included.