Don't use the Google Analytics "Search Engine Optimisation" data pulled in from Google Webmaster Tools. It's not accurate!
A post in a GA community got me looking into the GWT data it shows:https://plus.google.com/100518166975056212980/posts/3yHYQ1MoptE
I worked out that GA is still using the rounded versions of the GWT data, it is only reporting web based searches, and it is only reporting totals based on a subset of the real data.
Conclusion: Go straight to the source and get your data from GWT.
It also reports different average positions!
Here's the data for my website on a single day. If GA used the non rounded figures I suspect it would match up with the Displaying figures from GWT. The rounding is inflating it's totals.