Ranking will almost always differ because location has become a fairly important factor. The distance from the server to location of the searcher is a factor, as is the distance from searcher to whatever location is linked to the site.
On top of that, Google will look at past searches, and some browsers have location settings.
All together, these factors can make a massive difference. I found at some point that my site came up #1 on my own computer, whilst people 100 miles away didn't even find it on page 1.