utnik: Ah, damn, that's it exactly. I'd changed the "Preferences" rather than the "Document Setup". A school boy error. (There doesn't seem to be an emoticon for "Doh!".)
I'm now wondering if there might be a simple way of showing the user that each dialog is different in some way. Maybe a label at the top of the dialog telling the user where the changes will be applied, or something. Maybe it's just me but I don't go into either dialog very often - and I guess lots of users will be the same - so I keep forgetting the difference between them. Or maybe it's just a very simple thing that I'm just not "getting"; that wouldn't be a new concept by a long chalk!
Nermander: I think what you've said explains a bit of my confusion. There are a lot of "pseudo-DPIs" that can get in the way of the real info.
Here's another example of where I keep making a mistake…
I can create an image in GIMP which is 128x80 pixels but also give it a DPI of 720x720. GIMP has no problem with this so, on the surface and without giving it any thought, it sounds reasonable - it's a small image with very fine detail. But… just giving it a tiny bit of thought shows that it is clearly silly. On the horizontal you've only got 128 pixels so how can you have 720 "dots" in 128 pixels? You just can't, it's physically impossible to have more "dots" than you've got pixels. All you get is a really tiny image (which Scribus displays perfectly but it just looks very strange until you realise what you've done wrong). I've done this so many times it's becoming embarrassing.
Maybe it's just me getting old/stupid/useless [delete as appropriate], or maybe it's because I don't do this stuff very often (I only do DTP-related things a couple of hours a week on average, although I do try and visit the forum when I can), but I keep forgetting some very basic stuff so I keep making the same silly mistakes and have to re-learn things over and over again. Some things just don't stay in my head long enough to be permanent.
Anyway, thanks to you both for your, as usual, excellent answers. I just hope I remember so you won't have to give the same answers again a few months down the line!
Cheers.