There are a variety of reasons why this can happen. You could be connecting to a different Google server, you could have custom search results, and yes... search engines are starting to make the results more dynamic - so that is a possibility.

I have noticed that too. I am ot sure but I heard that it does depend on where you look results up from.

you can check your rankings on multiple Google datacenter using the following tool
Multiple Data Center Google Search - SEO Tools - Search Engine Optimization, Google Optimization

Although in the initial stages of getting your site ranked you could possibly see difference in your site ranking for the same keyword across multiple datacenters, over time the rankings should be consistent

And yes the rankings could be different across countries and your personal search preferences setting.