One small addition to sem4u's excellent observation:
If you set your Daily Budget at a level sufficient for your ad to appear 24/7, then you'll literally be capturing the actual number of impressions, for a given time period, for each keyword.
If your budget is lower, however, you will not be capturing all impresssions that occured - because there will have been periods of the day during which your ad was not showing.
And lastly, if one were to do this as a test, I'd also advise setting the Max CPC high enough for the ads to appear on the first page.
AWA
