It is possible that some of your ad impressions are not taken into account if our pixel is cached by the browser.
Once the pixel is cached, it is no longer executed from the server so we can't receive the request. In such a case, the number of impressions will be under-estimated.
We suggest that you add the following parameter to your pixel:
You will need to replace rand_num with a random number each time the pixel is displayed. It will force the browser to load the pixel each time as it will believe it is a different image.