I wasn't there two decades ago when the spec was first being developed, but based on other parts of the spec they're clearly attempting to establish a parallel with
/usr/local. The intended minimal
XDG_DATA_DIRS form a similar order for data files that, were it to be represented in a single environment variable like
PATH, would look like
Whether this was the XDG's idea or whether they were codifying an existing informal convention, I don't know. The use of
~/.config as the default for
XDG_CONFIG_HOME instead of
~/.local/etc suggests the latter.