When a wall post is formed by placing something on top, like a torch or glass pane, it has a different sized hitbox/collision box than a post created by having a solid block on the backside of the wall.  This can be seen when looking closely at the hitbox as one version outlines the wall exactly and the other hitbox sticks out 1 pixel further. More importantly, that 1 pixel is enough to stop you in your tracks if you are walking or travelling by boat against the wall.

Steps to reproduce:

1. Place a row of any wall block
2. Place torches on the first half turning them into posts
3. Place solid blocks behind the second half of the wall turning those into posts as well
4. Walk along this wall rubbing against it and you should get stopped at the 2nd half as if you ran into a wall