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The difference you see in numbers between Universal Analytics (UA) and Google Analytics 4 (GA4) for your subdomain pages appears to be excessively large, and therefore, it is understandably concerning.
To understand this difference, it’s important to note that UA and GA4 have different ways of tracking and counting sessions, users, and interactions. However, these differences typically cause a variance of around 10% – a far cry from the 90% difference you are noticing on your subdomain pages.
You’ve already verified that both UA and GA4 tags are firing correctly on your pages with Google Tag Manager (GTM). This indicates that GTM is working as intended, and suggests that the problem lies in the recording (or non-recording) of interactions on GA4 rather than with the GTM setup.
There could be a few technical reasons behind the significant difference in numbers, including issues like the improper configuration of GA4 for subdomains, the effect of same-site cookies, or even a difference in the handling of bot traffic.
In simpler terms, despite GTM correctly firing tags, it might be that these interactions are not being correctly recorded or categorized in GA4 when it comes to subdomain pages. It could be a good idea to closely examine your GA4 configurations and ensure they are in line with best practices, especially for subdomain tracking. Also, ensure that you’re comparing equivalent metrics in GA4 and UA, as GA4 might now use different definitions for previously familiar terms. Additionally, seek help from technical forums or GA experts to dig into more specific issues.
