Why isn’t the GISJOIN the same from one decade to the next for a census tract that doesn’t seem to change?
The numbering of census tracts (and other lower levels of geography like census blocks) is determined systematically for entire counties by a combination of local and Census Bureau authorities. In the early years of tract definitions, the numbering systems occasionally underwent dramatic revisions, but even in recent censuses, when numbering systems have been more stable, tract boundary changes in one part of a county can force a renumbering to tracts throughout the county to accommodate a more logical numbering.