Fences attached to any block elevate you by a half block

So I was making a stall for some horses and other related mobs, and I noticed that my Llama had escaped. I checked to see how,and it had apparently somehow climbed  onto the attached part of a fence post, and jumped onto a half slab I had put on the edges of my enclosure. Not sure why fences behave like this, nor how long it's been an issue, I can't remember necessarily experiencing this before.

Keep up the good work!