Recently the DPI laws have been trampled, my friends. Evil sneaking cunning that goes about it's way and claims to have a higher DPI than so. Take some printers for example. Some will claim to have twice the DPI of what they're capable of because instead of moving the width of the printer head across, they'll only move half that distance. For example.
Say we have a regular DPI grid in which the printer head can fill each box and gradually composite an image:
+---+---+---+
| | | | <- paper
+---+---+---+
| | | |
+---+---+---+ +---+
| | | | |XXX+ <- printer head size
+---+---+---+ +---+
The printer's printing head hovers over the paper. With the limit that the smallest physical mark it can make on the paper is a square the size of one of those boxes in the grid. That's the smallest mark the printer head is built to make. But what if the head - instead of moving one printer head unit along - moved along only half! That would "double" the DPI. Rest ye weeping soul here friends!
+-+-+-+-+-+-+
| | | | | | | <- paper
+-+-+-+-+-+-+
| | | | | | |
+-+-+-+-+-+-+ +---+
| | | | | | | |XXX| <- printer head size
+-+-+-+-+-+-+ +---+
So now above we have a grid with double the density - even though the printer head is the same size. You don't have a grid in the normal, regular, wholesome way we've grown up with. It's a kludging with twice the horizontal dots but the inability to do a single dot any smaller!
Argh! the devil lurks!
Thus you DO get a higher quality print, but not of the magnitude expressed by the DPI value. It's a rectangular composite grid - but you have to fill in two boxes at once!
red rum red rum, argh!
(ps. I know "printer head" isn't the right term, but it might be the easiest to explain)