The sample openacs-config.tcl file from naviserver [1] is setting per default the db_user to the name of the server instance - similar to the sample config file for aolserver and also similar to what you describe.
The install script install-oacs.sh [2] creates among other things the oacs_user with the configured value in postgres and alters the default db_user that value (normally "nsadmin"). This happens in nstall-oacs.sh around line 310. So, an explanation might be, that install-oacs.sh was not run (has not tailored the startup file) or stopped prematurely.