BigQuery - Android Purchase Attribution

Question

I'm trying to replicate GA4/Firebase traffic & user acquisition reports for mobile apps using BigQuery.

I'm using this simple query to check the number of users, new users and purchasers for a specific day.

 select
  traffic_source.medium,
  count(distinct user_pseudo_id) users,
  count(distinct case when event_name= "first_open" then user_pseudo_id end ) as new_users,
  count(case when event_name= "purchase" then user_pseudo_id end ) as purchasers
from
     'mytable.events_*`
 where platform='ANDROID' 
 group by 1
 order by 1

Here are my BigQuery results:

However, they do not match the data from GA4 explorer reports:

The number of new users matches well, however, the distribution of purchasers is completely off.

Does anybody know what the issue might be?

Many thanks:)