Are sessions different in Google Analytics 4 and Universal Analytics?

Is this happening with someone else? My data in sessions is completely different for GA4 and UA. Please help!!

Sessions in GA4 and UA are not adding up because, with GA4, Google has changed the definition of Sessions. Here is how -

1. In UA, a session is a defined period of time when a user is actively engaged with your app/website. It has got set parameters for it to end -
   --> Session ends when 30 minutes of period inactivity (depending upon session timeout setting) is encountered. If a user comes back after this inactivity, it will be counted as a new session by UA
   --> On the timestamp (depends on the time zone followed by that view). It means if a user is on your website, and then midnight comes, then post-midnight it will be counted as a new session in UA
   --> Change in campaign parameters. This means if a user comes to your website via two different campaigns or organically, each of these visits will be attributed to a different session

However, in GA4, a session starts with a session_start event. This generates a session-id. This helps you determine the session that is responsible for subsequent events because GA4 will associate this session_id with each subsequent event in this session. Here are a few more changes to how sessions are counted in
GA4 -
--> A session will end after 30 minutes of inactivity (depends on session timeout settings that are deployed) . If a user comes back after timeout, a new session is counted
--> Even if midnight is encountered during browsing, a new session is not counted
--> When coming to the website with new campaign parameters, a new session is not counted.

Having established these differences, these are some of the reasons you are likely to see a different session count in GA4 and UA:

1. Timezone - If your website has a global customer base, they may cross the midnight timestamp depending on their geographical location. Thus, session count will be different for GA4 and UA

2. Using UTM - Different parameters will lead to different sessions in UA. So avoid using UTM within the website. However if you do, then your UA session count will be much higher than GA4 session count

3. Modeling and Estimation - GA4 is based on a model that facilitates tracking when the user goes cookieless. Thus, it uses estimation and behaviour modelling to estimate session count. UA uses no such estimation.

4. Using Filters - In UA you have filters that may exclude some data. In GA4(360) reporting, you can alter which data from a property is available in subdomain/subproperty. But even if you filter out session_start event data, a session-id is still generated for a subproperty in GA4.

These are the differences and why session count is different in GA4 and UA. Hope it helps!