See also the definition of a click.
The ad delivery systems reports the number of clicks on an ad, but there is also possible to get the same metrics from web analytics, like Google Analytics. Almost.
The ad delivery system and the web analytics reports the same metric.
While the ad delivery systems reports the exact numbers of redirects from the ad to the landing page, the web analytics reports the traffic originating from the ad.
But why isn't the ad delivery system and the web analytics reporting the same numbers?
Visits and duplication
One possibility for the discrepancy is that the ad delivery system and the analytics tool isn't reporting the same metric. While the ad delivery system reports the actual clicks, the analytics tool is possibly reporting visits only.
So what if the same user clicks on the ad twice? Will the analytics tool count this as two visits or just one visit? Maybe you could check if the analytics tool can report the number of pageviews that comes directly from the specific source?
Referer or tagged URL
And the analytics tool must know exactly where the traffic comes from. This shouldn't be done with the generic referer method. The ad could be displayed at my-fancy-blog.net, but the ad itself could be hosted (in an iframe e.g.) at xyz.123.ad-delivery.savvy-adnetworks.com.
Would the web analytics understand that these are one and the same?
To make sure the web analytics knows where the traffic comes from, the URL should be tagged properly. With Google Analytics this means the URL to the final landing page should contain parameters like utm_source, utm_campaign etc.
And even though you have added tracking parameters to the URL, there could be a discrepancy. This could be the case if your parameter values contains any odd characters.
Make sure you keep away from any special or local characters, and stick with the letters A to Z, the numbers 0 to 9 and hyphens or underscores.
Never arrived at landing page
There is also a possibility that the visitor that clicked on the ad never arrived at the final landing page. The visitor could close the window, or go back in their browser, before the landing page has loaded.
If the user is reading an article from a wireless device (a phone), and after a while sees the ad and clicks on it, the user could in the meantime be in an area without any reception. This could very well be the case if you are on the subway with limited reception.
I also recommended using the CPM calculator to make sure you get your numbers right.