Why are there discrepancies between Adwords and GA data?

When you've finished integrating Google Adwords with Google Analytics, you will notice that the number of Clicks reported by Adwords does not always result into Visits.  In simpler words the Clicks do not match Visits metrics.  You will notice similar data like the below screenshot. 

There are several reasons for these differences. First thing to remember is that Adwords tracks "Clicks" whereas Analytics tracks "Visits". 

Second, it is possible that some visitors who click on ads may have JavaScript or Cookies turned off. You may wonder how do these two cause discrepancies in the data. 

Let's take JavaScript first. Once the user has JavaScript turned off in their browsers, they are untraceable in Google Analytics, so you will not only loose the data on who is seeing the correct Adwords ads, you wont even know that they visited your site in the first place. If you are noticing that your leads are low, you might want to check your web server logs, and compare the number of visitors to your landing page to the number that Analytics shows. This would at least give you an idea of how many people you aren’t tracking in Analytics, and since the reason is likely that their JavaScript is turned off, you would also know that they didn’t see your Adwords ads. 

Now coming to Cookies. A Cookie is a snippet of text that is sent from a website’s servers and stored on a web browser. Like most websites and search engines, Google uses cookies in order to provide a better user experience and to serve relevant ads. When a cookie is set on your browser, it allows Google to gather information about your browser's interaction with a given ad. So if you don't want Google to get detailed information on what ads you clicked then you can click the "opt-out" button. Note: This opt out is only specific to the browser which you are using. 

Analytics uses its own set of cookies to track visitor interactions. These cookies are used to store information, such as what time the current visit occurred, whether the visitor has been to the site before, and what site referred the visitor to the web page. To disable this type of cookie, some browsers will indicate when a cookie is being sent and allow you to decline cookies on a case-by-case basis. 

You’ll also see differences between Analytics and AdWords if the Google Analytics Tracking Code on your landing page doesn’t execute. In this case, AdWords will report the click but Analytics will not record the visit. 

Invalid clicks is another point for data discrepancy. Invalid clicks may also cause reporting differences because while Google AdWords automatically filters invalid clicks from your reports, Google Analytics will still report the visits. 

Keep in mind that the adwords data is uploaded once a day to Analytics so the results for each may be temporarily out of sync. 

There are some other common issues too:

a) Sometimes Destination URLs or in other words PLPs are not tagged.  Make sure that auto tagging is enabled in adwords otherwise your visits will be recorded under organic and not CPC.

b) If your site uses redirect then campaign data can be lost.  Redirects often obstruct the GA tracking code from launching. Therefore the visit will not be recorded as coming from Adwords, but Adwords will still report the clicks.