it's a UI flaw
Don't they stay in same place if you sync the updates from itunes rather than download them to your device? I've never noticed them changing around when iTunes populates updates. Not a perfect solution, but it works for me.
Kind of ruins the whole point of the app store though.
Apple actually has a UI "bug" of sorts there as the behaviour makes no sense and violates it's own UI guidelines in a way.
When you use the hardware button to turn the iPod/iPhone on and off, it defaults to the last of the various "desktops" or whatever you want to call them that was in use. This suggests that there is no "special" one.
The fact that all downloads go to the "home" screen, suggests that it is indeed "special."
It should really be all one way or the other. Either the home screen is "special" and pressing the hardware home button should always return you to that screen, or it isn't special, and the apps should "know" what screen they belong to and download updates to that location.
Highly illogical as Mr. Spock would say.
The most sensible course would actually be for both to be changed. No one wants to be moving icons around all the time, but the hardware "home" button should still really take you to the same place all the time. That's the whole purpose of the button.
I am always a little surprised with each update, that Apple still hasn't seen this flaw in the logic of the UI.