The differences you're seeing are almost certainly due to hitting different datacenters, as mentioned by Trainer Bubble.
Personalization, as mentioned by Sporkman, may also play a factor, depending on whether you're logged in to your Google account, whether you've enabled or disabled search history, whether you're doing all these searches from the same PC and whether you actually click on any of the results or just run the searches.
Of course, both those factors will also affect everyone else who's searching, too. Which means that even if you see great rankings for your page, it may not necessarily mean that anybody else on the planet sees the same thing you do.
Which sort of begs the question: if a page has a #1 position, but the only person who can see it is the site owner, does it make a sound?
I've found the best thing to do to avoid heartburn is to stop checking rankings almost entirely. I haven't checked rankings for my day job company in at least three or four months.
I do, however, check our traffic
every day. As long as the traffic trends are going the way I want them to, and the search terms people are using to find our site are those we expect, I don't worry too much about what any individual datacenter might be showing for rankings.