Web advertisers use these measurements to determine how and where to spend their money. Ummmm just one little thing new research reveals those measurements are almost always wrong. Since several different factors contribute to this inaccuracy, it's impossible to know whether any given ad count is too high or too low. Several factors work to artificially inflate ad counts, as Michael Tchong related in a recent edition of his email newsletter Iconocast.
Visitors become impatient and double or triple click the page or banner.
Some ad-counters filter out these extra clicks, but many do not. Hits generated by software, not by ad-viewing humans. If a site is overly conservative in counting ad impressions , then click-through will be artificially inflated. You might assume it is more effective than it really is. Meantime, other factors deflate ad counts, especially caching.