There are lot's of variables that affect the displaying of ranking on the SERPs, some of them include location, account settings and many other things. Google also runs tests on their search results constantly so you might see some variation from that as well.
What does all this mean? Don't worry about the details so much but look at the overall trend to see if in general your rankings are going up or down. If you are tracking rankings manually i would use the same browser using the same settings each time to check the rankings, this you will use as your baseline. Then you will be able to monitor for any changes in rankings based on these set variables. If you then check other browsers you should see the same general trend even though the results may not be exact.