I've just noticed in one of my ad groups I have Widgets and widgets as 2 separate keywords - Widgets has had 12 impressions and 4 clicks (average position 1.0) ... widgets has had 24 impressions and no clicks (average position 1.1) I wasn't aware of having two of the same keyword until earlier today, but I assumed Google always treated the keywords as not case sensitive?
From what I understand, case sensitivity is not recognised. Hover your mouse over each keyword's Ads Diagnostic Tool shortcut (the small magnifying glass) and see what messages are presented. I'm assuming that, at any one time, one of the keywords will not be triggering ads. You will get a message saying something like: "A keyword in another ad group is triggering this ad." The keyword which works will change occasionally depending on whichever one has the highest quality score. This is why you have observed impressions for both keywords.