Inconsitancies in pg_stat_bgwriter and pg_stat_database returned values

Hello, 

On one particular PG12.3 instance (same behaviour with PG12.4) we are experimenting occasional strange values in the
abovetables. 
 

For instance looping (10s. period) over this request: 

SELECT current_timestamp, checkpoints_timed, 
checkpoints_req, 
checkpoint_write_time, 
checkpoint_sync_time, 
buffers_checkpoint, 
buffers_clean, 
maxwritten_clean, 
buffers_backend, 
buffers_backend_fsync, 
buffers_alloc 
FROM pg_stat_bgwriter 

Gives: 

020-09-16 13:41:39.781609+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:41:49.815612+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:41:59.851812+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:42:09.89053+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:42:19.923291+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:42:29.959282+00,3291,227,1278516493,26327,66708986,1743379,16800,253585756,0,59080296 
2020-09-16 13:42:39.98198+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:42:50.028727+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:43:00.067928+00,3291,227,1278516493,26327,66709285,1743379,16800,253585756,0,59080296 
2020-09-16 13:43:10.090266+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:43:20.140272+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:43:30.170372+00,3291,227,1278516493,26327,66709585,1743379,16800,253585756,0,59080296 
2020-09-16 13:43:40.204747+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:43:50.239374+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:44:00.26905+00,3291,227,1278516493,26327,66709884,1743379,16800,253585756,0,59080296 
2020-09-16 13:44:10.314927+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:44:20.351091+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:44:30.408797+00,3291,227,1278516493,26327,66710184,1743379,16800,253585756,0,59080296 
2020-09-16 13:44:40.44833+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 
2020-09-16 13:44:50.484298+00,438,2,10095976,0,611207,870625,8695,612255,0,39028850 

One can see that some columns that are supposed to increase only are not always. For instance checkpoints_timed
suddenlyjumps from 438 to 3291, then back to 438. 
 

We experiment the same issue in pg_stat_database. 

Thanks