This is a common issue that can arise due to several factors, primarily revolving around how data is collected and processed by these two systems. 

Client-Side Script Blocking

As you may be aware, GA4 relies on JavaScript, which runs in a site visitor’s browser. This is susceptible to interference from various sources:

• Ad Blockers and Privacy Extensions: Many users employ ad blockers or privacy extensions in their browsers. These tools often block scripts from third-party platforms - including GA4 and other well-known analytics tools - preventing it from recording visits or conversions.
• Browser Privacy Settings: Modern browsers have heightened their privacy controls. Some, like Safari and Firefox, have default settings that can restrict the tracking capabilities of third-party cookies and session-based tracking methods. They also utilize an opt-in method, rather than opt-out, to further improve a user’s data privacy.

Differences in Tracking Methods

GA4 and your payment processor use different methods for tracking conversions. GA4 primarily uses JavaScript and cookies to track user interactions on your site, which is contingent on the user's browser allowing the script and cookies to function. Applications like cookie walls and other blockers can prevent these scripts from executing properly.

Your payment processor likely tracks conversions directly at the server level immediately when a transaction is completed. This method is generally more robust and less likely to be affected by client-side issues. In fact, this should be 100% accurate.

Other Common Reasons for Failure

Network Issues and Latency: Sometimes, a user's poor network connection can result in GA4 scripts failing to load or execute properly, leading to untracked visits or conversions.

Cross-Domain Tracking Challenges: If your conversion process involves multiple domains (for example, moving from your main site to a separate payment gateway), tracking across these domains can sometimes be mishandled or lost if your property doesn’t exist on both.

User Opt-Out Choices: With growing awareness of data privacy and more jurisdictions establishing strict guidelines for user protection, more users are opting out of tracking. GA4 respects these choices, which can result in underreporting.

Measuring the Discrepancy

Ultimately, the variance of the data discrepancy between these two systems can vary greatly depending on technology used by site visitors and purchasers. It’s important to note that achieving 100% data accuracy in analytics systems is virtually impossible. These systems are meant to be used as tools to inform decisions that lead towards optimization and growth, and those visitors who agree to share their information with you are doing so freely. However, user privacy and data retention is of paramount importance, so it’s not unheard of to see discrepancies of 40% or more.

Final Word

While GA4 provides valuable insights into user behavior and website performance, it's important to remember that its data might not always align perfectly with server-side systems like your payment processor. This discrepancy doesn't undermine the validity of GA4 data but highlights the need for a multi-faceted approach to data analysis. Using a service like GA4 which respects a user’s preferences will help you align with accepted practices.

We recommend using these insights collectively to understand your audience better and make informed decisions. If you have any questions or need further assistance, please feel free to reach out.