Lumia 920 has an Ips display, so that's why pure motion can be added.
On amoled displays this cannot be added because ips and amoled's are functioning very different.
Basically ips it's an lcd backlitted display. Altough the refresh rate will be let's say 60hz, and the screen will refresh on this rate, the light will stay continuosly on in the back, so only the display itself will refresh.
On the amoled, because it's not backlitted, and every pixel it's basically a led ( a very small one), not only the screen will refresh but the pixels will turn on and off independently 60 times per second.
Here is the difference, it's easier on lcd to do that, because o pixel will change only it's colour, and the light will stay on. Even on black, the light will stay on behind the lcd. On the amoled, the pixel will completely turn off.. so try to imagine if you have a light bulb that can light in different colours, and will change his colours 60 times per second, how it will look when the colours will change without turning the bulb off and on?? It will be smoother, right?!
Now try to imagine, how it will look if you will turn off and on the bulb, everytime you want to change the colour...not so smooth... That's why 930 doesn't have Pure Motion..
But trust me, on a phone...it's not a big downside.. On my tv, I have 800hz refresh rate..this is very cool..on a mobile device, not as cool as those deep blacks that amoled provides.....
But in the end, both screens are good...it's a matter of taste...
I am dissapointed by the fact that 930 does not have glance screen(Altough it's possible- tested by me) because the screen is capable of that(ips screen are not)...