[BeginnerQuestion]Why these is 6 rows in my SELECT statement?

[BeginnerC]

Hello community,
I am checking my pg_stat_activity view,but something confused me.
Just like this:

postgres=# SELECT wait_event_type, wait_event FROM pg_stat_activity;
wait_event_type |     wait_event
-----------------+---------------------
Activity        | AutoVacuumMain
Activity        | LogicalLauncherMain
                |
Activity        | BgWriterHibernate
Activity        | CheckpointerMain
Activity        | WalWriterMain
(6 rows)

As you can see, the third row is empty,but I don't know why.
Can anyone provide some solution to me?
Thanks in advance!

Yours,
BeginnerC.