Long time ago, trainers were accepted for author status. Now it's only for F.A.Qs/Guides. I think this happened because many people know how to make trainers.
Tiger Woods PGA Tour 08
(PC) Trainer, 9/7/2007
PWiz and Nev were accepting trainers for authorship but as of 2008, only FAQs and guides count towards the free unl. membership. Dunno why, you should ask them.
I think this was an easy way of getting unl.membership. So to prevent CH from getting a rip-off, they change it to FAQs, which was not so easy to write.
You are getting the wrong impression. What I was trying to show is that Trainers are easier to make than FAQs. Of course, there are certain exceptions.