Allright then. The 10.5.4 combo upgrade is an actual upgrade, in that it does not contain all the files needed to run OS X. The retail DVD is not an upgrade, in that it contains all the files needed to run OS X. If some company chooses to discount a full version because you own a full previous version does not make that full version an upgrade.
10.5.4 is not an upgrade, it is an update. Period.