I'm sure that some poetry is written deliberately to be undecipherable. Other work is probably for an "inside audience", maybe for a competition of sorts or one-upsmanship. Who knows what they mean. Poets are human and have egos and sometimes are a bit nuts (aren't we all). Sometimes only half of the story is told (just can't say it staight). I think some develop a style that evolves away, maybe way away, from common language. Maybe only the poet knows at that point. A lot of what I have written isn't meant to be read by anyone else. I need to talk to somebody I really know and trust (me) and I need to hear from all those other folks (my other egos). It gets complicated. I got royally turned off to poetry at one point when a young prof. tried to tell me what some dead poet, whom he had never met, really meant to say in his poem. Go figure.
One of the problems with poetry as an art form is that it is language based, so you have to know the language, unlike music or sculpture, where you really don't need translation. And, since language becomes the artistic tool in poetry, it may be manipulated all over the place for effect. That gets hard to follow.
I could go on, but enough for now . . .