pg_stat_database.xact_commit + pg_stat_database.xact_rollback as areliable metric for xid consumption ?

Hello
I attended two interesting online courses by 2ndQuadrant recently, about MVCC and Freezing , and I started to seriously
diginto this for the first time in our almost 20-yrs postgersql installation 
 
(running 10.x currently).
I thought of using the info from the statistics collector : pg_stat_database xact_commit and
pg_stat_database.xact_rollbackbut to my surprise this gave very high numbers, almost 20 times up , so I 
 
wrote a cron to insert every morning into an archive/historic table : pgstat_database_arc which is basically the schema
ofpgstat_database plus a timestamp col. And I got two instances of it one 
 
yesterday and one this morning :

dynacom=# select xmin,datname,xact_commit+xact_rollback as num_of_xacts,ts from pgstat_database_arc WHERE
datname='dynacom';
    xmin    | datname | num_of_xacts | ts
-----------+---------+--------------+-------------------------------
  813560437 | dynacom |   7734784237 | 2020-05-31 18:37:08.950399+03
  813889272 | dynacom |   7740424341 | 2020-06-01 08:00:01.912157+03
(2 rows)

But the reported increase in the total number of xacts is 7740424341 - 7734784237 = 5640104 , while the actual xid
consumption(using xmin, this table only does inserts) is : 813889272 - 813560437 = 
 
328835. The xid difference (via xmin) is compatible with another system that I maintain (for our custom replication
solution),so I am wondering why this almost 20-fold difference between actual xid 
 
consumption and pg_stat_database metrics.

-- 
Achilleas Mantzios
IT DEV Lead
IT DEPT
Dynacom Tankers Mgmt