The impressions and clicks don't seem to correlate on Google Webmaster Tools for my site. Whereas the impressions vary widely, the clicks stay constant. I would expect the clicks to my site to go up (even if slightly) when the impressions go up.
Sample days: 2,522 impressions, 91 clicks (3.6% CTR). 1,038 impressions, 54 clicks (5.2% CTR). 2,472 impressions, 104 clicks (4.2% CTR). 1,123 impressions, 70 clicks (6.2% CTR).
So whenever the impressions get less, the CTR rises a lot, when impressions get more, the CTR drops. I have only given an example of four days but this behavior is consistent throughout the 90 days that GWT shows.
It will probably be tempting to put this down to chance, but looking at the graph on GWT it seems not to be a matter of chance. The impressions line varies between 1000 and 2500, with huge jumps up and down, whereas the clicks line stays completely flat, around 70-100.
Why is that?