How sessions track, GA4 and BigQuery

Hi,

Struggling to understand the relationship betwen the the ga_session_id in BigQuery and the Sessions in GA4.

For a specific low traffic page, I counted the ga_session_id (with pseudo_id) and got nearly the same session number as in GA4.

However, when I look at sessions across the whole site the number is significantly higher than the count of ga_session_ids.

This is all meant to be unsampled and alot of events/page views seem to not have an attached ga_session_I'd.

Any ideas how this works and relates?