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The discrepancy you are experiencing could be due to a number of reasons. One possibility is that GA4 is categorizing what should be ‘app’ and others as null on BigQuery. This issue could be arising because some referrers don’t pass the referrer information to your website and as a result, GA4 records the event as direct traffic or null referrer in BigQuery. Besides, GA4 and BigQuery handle bots and spam sessions differently and this could also result in discrepancies.
Another important factor you need to consider is the timing of your data export from GA4 to BigQuery. If the data is exported in real-time, there could be some differences due to data latency. Therefore, it is recommended to compare data of GA4 and BigQuery for a past period, when both datasets are complete.
If you are fetching the UTM parameter from the URL, you have to consider that the page_location variable captures the full URL of the page where the event is triggered. It is possible that not all your sessions start on a page where UTMs are present in the URL or maybe the UTMs are dropped in subsequent pages within the same session. Therefore, the actual number of sessions per campaign in GA4 can be different compared to what you get when you fetch the UTM from page_location in BigQuery.
It’s also worth checking that the event parameters that you are using in your SQL query do indeed contain the referrer data you need. Use DebugView in Firebase to see the events and their parameters as they occur in your GA4 property, and make sure you’ve got your query parameters correct.
Finally, remember that the way GA4 and BigQuery treat and record data is fundamentally different. GA4 preprocesses and aggregates data for you, while BigQuery provides the raw, event-level data. Discrepancies can arise simply because of these inherent differences. It’s always a good idea to thoroughly understand the data schema of both GA4 and BigQuery, and know exactly what each field contains and how to use it.
