Zombies can't jump up from a grass path to a block.
Look at this video:
https://youtu.be/g2l63MRmims?t=8m30s
It's german but I think you can see what I mean.
Environment
Java: 1.8.9_25 64bit
OS: Windows 10 Home
CPU: 4x Intel Core i3-4170 3,7 GHz
GeForce GT 730
Zombies only jump one-block high. A path block is slightly smaller than a normal block. So this is normal. Otherwise you'd have zombies jumping at 2 high walls.