Why are user counts ~30% higher in GA4 vs. GA Universal Analytics?

We have a website that has Google Tag Manager (GTM) installed on it. We have a pageview tag that sends data to GA Universal Analytics, and separate pageview/configuration tag that sends data to GA4. Both tags use the exact same trigger logic. When simulating in GTM’s ‘Preview’ mode, I’ve confirmed that 1 pageview correctly fires for each page (the GA tag and GA4 tag each fire once per page). However, our GA4 user count is consistently about 30% higher than in GA.

I’m at a loss as to what could be causing this. Any idea what it could be?


I’m sure if you contact Google a clear answer will be soon to follow. :grin: