Explanation needed?

[Reuben Thomas]

In the node "Installation Directory Variables" the following makefile 
fragment is given as an example:

     edit = sed \
             -e 's|@address@hidden|$(pkgdatadir)|g' \
             -e 's|@address@hidden|$(prefix)|g'

In the second line, why is datadir replaced by pkgdatadir? I know this isn't 
relevant to the example, but it caused me to wonder if it was a typo and 
check that the real code really does contain such a substitution. But it's 
still not obvious why, as datadir really isn't the same thing as pkgdatadir!