Hello Karlos, that usually happens when your system receives a new email and the original ticket is closed and queue setting is to create a new ticket. In that case it creates a new one with reference to the old ticket in the subject and from there it will be part of the subject unless someone edits it.
The other ones refer to regular tickets with a normal subject, if you see all the ones with the ticket number are referring a reply email.
To know if there is such config, look into the option FollowUP in your queue settings, it should be set to "new ticket"
Best Regards