I've been working on making pretty jump tests for horses and have discovered that when stacked walls do not seem to be even 1 block high, let alone the 1.5 they're supposed to be on any middle parts of the wall.

A horse that can jump 2.5 blocks, but not 3 blocks when made out of full blocks and slabs or fences can still jump over the middle of walls stacked three high. A donkey, which can only jump 1 9/16th blocks can still jump over walls stacked two high.

Any wall that has the raised pillar part (the end caps, or if for example a string is placed on each middle part) goes back to being taller.

From my testing, it seems that a stacked wall is without a pillar is half a block high and causes the wall beneath it to be 1 block high, therefore three stacked walls are 2.5bh in the middle and 3.5bh on the ends. If I add a string on top, to turn each middle part into a pillar, the whole wall again becomes 3.5 blocks high as expected.

I haven't been able to find any information about this being intended or even known. 

Attached a screen shot that hopefully demonstrates the issue clearly, everything in the image is the equivalent jump height, except for the gray parts of the wall.