According to Google:
"Different analytics products may use a variety of methods to collect data about your traffic, so it's normal to see discrepancies between reports created using different products. But the best way to think of metrics across different analytics programs is to think in terms of trends, as opposed to numbers themselves."
NOTE: Originally, this was posted in the Analytics help site, but the post was taken down
Zeald reporting happens server-side and reports hit on our server. Google Analytics, on the other hand, run inside the client's browser. These differences mean that Zest has access to some data that Google doesn't and vice versa.
Analytics and ZEST potentially use different methods to determine a 'visit' and a 'visitor'. A visit is defined as a set of page loads on the site that happen within 30 minutes from a single user. We determine a 'visitor' by their cookies, which essentially identifies a specific device and browser. In google a 'user' is also able to be tracked using attributes like their google account across multiple devices (Google provide a rather vague description of how they track users here but in the end it uses a 'secret algorithm', so we can't be sure).
Analytics can in some cases more reliably detect 'bots' than the zeald reports (which also do an element of bot detection, but using different techniques that can be thrown off by certain nasty spam-bots). In that case you'll notice a surge of traffic that isn't in GA
Analytics can also 'miss' visits because the page closes too quickly for it to track them.
In things like 'goals', analytics will report 'unique page views" rather than "page views" (which ZEST analytics reports). ZEST also counts all loads of the page that show that page content, rather than analytics which only count the specific url, which may mean ZEST counts higher due to showing the page 'twice' for things like a failed form submission.
If your website does not have its own SSL certificate, then Google Analytics' techniques for tracking visitors break down as the user transfers between www.yoursite.com and https://secure.zeald.com. Zeald applies so-called "cross domain tracking" using the method recommended by Google. But this doesn't work well and Google will "double count" visits for at least some of your visitors, if you use this scenario. Since Google also loses track of some of the users as they move between domains, this can cause problems on Google's ability to track other more complex metrics, such as time on site, entry and exit pages, and so on.
SOLUTION: It is therefore recommended that you purchase your own SSL certificate for your site. Getting your own SSL certificate guarantees that these 'cross-domain' related issues won't happen. See Get Your Own SSL Certificate for more benefits of purchasing your own SSL certificate. If you would like to purchase your SSL certificate from the Zeald Portal, click here for more information.
In Google Analytics, 'goals' are defined in terms of loading a specific page in the frontend of the website. In ZEST many goals such as enquiries have their own database systems and can differ in a similar way to Ecommerce tracking due to manual backend changes. Analytics goal tracking can be thrown off if you don't have your own SSL, as explained above under 'Visitors Numbers''.
For more accurate reporting we strongly recommend the use of both systems in parallel, and that you purchase an SSL certificate. It will be more difficult to compare figures between the two systems and we suggest that you should focus on trends as opposed to specific numbers