What do the mysqlds do in my mysql database?
I see 18 mysqld's there.  I'll bet SHOW VARIABLES LIKE 'thread_cache_size'; gives you at least 18.  And, I'll bet that, at some point since startup, you had 18 clients running.

So why doesn't PROCESSLIST show 18 processes/threads?  Because wait_timeout killed off 17 of them.

Bottom line:  Nothing to worry about.