Its simply the expense of the path. It's not that barbed wire was added, it was that making it a more hp expensive pathway resulted in a different pathway being chosen as the cheaper one. They will always flow along the path of least resistance as their base movement rule but faatal is adding in some factors that cause some of them to forget the optimal path so it won't be quite so apparent.
And this is something I fundamentally disagree with.
They should generally try and take the path of least resistance, yes.
BUT there are two things that NEED to be implemented, or this whole system is worse than ever:
a) they must ignore spikes and other traps. ANY of them. Not a single point for traps. They are mindless zombies with no sense of preservation. Traps are not "do not tresspass" signs. They are traps designed to catch the unaware. Be it animals in reallife or zombies in game.
b) if there is no direct line of sight, they should not know what lies behind.
What I mean by that is the following:
1_| || ||| ||||
2_| __|||| ||||
b ||| ||| __ | ____Zombie_____
4_||||_|_| |||
5_||||||||||||
its a bit confusing, but I dont want to start painting now. (basicially every | is 500 hp where every block can have |-|||| hp.
If the zombie has a as its goal, it should NOT go for the 2nd path, even if it is the easierst one. It should go for the 3rd, then soo that 4th is easier to break, continue on 4 and then goup again as they hit the 4 hard wall. (I know its hard to understand, but im really REALLY lazy

)
NOW can you program in that "just hit randomly" thing. Then someone might "accidentially" break a stronger block that opens up the path more, but not because they know whats behind.
Basicially TL;DR:
You can calculate the route they SHOULD take/which are possible, but only ever give the zombie the information they have (so in sight, maybe around small corners) never more. (maybe on bloodmoon, but even then point A and B still should apply.