Wilburforce - 1:29 pm on Mar 14, 2013 (gmt 0)
surely the traffic would reflect these positions even if I can't see them?
The Impressions, Clicks and CTR columns will give you a better idea of how a particular term ought to affect traffic, but those data are for the whole period, whereas Average Position could derive from single DC for a single hour within that period. Google are not exactly forthcoming, but the one thing I do know is that anomalies between WMT position data and what I actually see when I search are very common.
There was an earlier thread on it which probably won't shed much more light: [webmasterworld.com...]