Whenever any member of the development team makes any change to even just one line of code, it is assigned a different build number. Sometimes an interim build will be assigned to separate teams and there will be a skip in build numbers for one of the teams. (for instance at build number 3000, the process may be assigned to two different teams and one team will continue with build 3001 and the second team will skip to build 5001 or such. This would prevent duplicate build numbers)
To answer the question posed in your post title: NO, 7600 is NOT a release candidate. Build 7600 is RTM (release to manufacturing) which is the FINAL version that will be pre-installed on computers that will be on sale on Oct 22 and is the version that will be in cute little plastic boxes on store shelves on Oct 22. This version is also being "leaked" out a few days early through many retail outlets both pre-installed on new computers and in plastic box versions this week. Microsoft has "blessed" this early release in certain circumstances, but October 22 is still the "official" release date.