I believe you, It's just that my info is out of date! (story of my life)
Actually the more I think about it the more it does make sense. Say a station has 2 crews, and engine crew and a ladder crew (or squad crew). The engine is more economical to operate, so they get most of the calls. But then that crew is busy and the other crew sits around waiting for a call that uses their equipment. So why not send them out too? But it's not economical to send out a big ladder or Squad, so they send the crews out in smaller tender trucks.
Have you ever looked at this site?
Phoenix Fire Department Apparatus