Google instant uses a different data set than the "hit enter" variety. I've been noticing that when I write an article I can see the ranking results within 30 seconds if I type in the keyword and hit enter. If I just type in the keywords and do not hit enter Google instant shows me what the rankings USED to be. In short the "enter" variety are instantly updated but Google instant isn't. Ironic but hey...
I've also noticed that I can gauge what the non Instant rankings will be within seconds of posting an article(yay for fast indexing). As such it's possible to use Google instant as an almost instant preview of changes to rankings, just hit enter and you'll see any changes in the top 10 immediately.
Hows that for a fun but fairly useless tidbit of info? :-)