The section of the combat example that you are looking at says the following:
[The zombie] does Str+d4 with its claws. It’s Strength is a d6, so the
GM rolls a d6+d4 and gets a 5. Dan’s Toughness is 5, so he’s Shaken.
In this case, the d6+d4 is the result after you replace Strength (Str) with the Strength of the zombie, which is a d6.
However, you are keen to note that the bestiary zombie's claw attack only does Str damage (in other words, just the zombie's d6 Strength), rather than Str+d4 as is in the example.
The opening for the combat example says that it is in a "Weird Wars: Vietnam campaign", which would mean it's part of the Weird Wars: Tour of Darkness setting. Interestingly, that book has zombies with a damage of Str+d4*, which matches the example.
Looking at a few other books as well, Pinnacle seems to flip-flop between the damages. Deadlands Noir has zombies with a damage of Str while the Horror Companion has zombies with a damage of Str+d4.
So what does this mean? Probably just that the editor didn't notice the difference between the two books and didn't make the connection. So I'd say you can use either one as you see fit, depending on how lethal you want zombies to be.
*Actually, it's listed as "Str+1", which is how melee damage was written in Savage Worlds Revised. When the Explorer's Edition came out in 2007, damage formulas were changed and Pinnacle said that pre-EE settings should be converted where "Str+1" becomes "Str+d4", "Str+2 becomes Str+d6", etc. It wouldn't surprise me if whomever created the example was looking at an updated version of Tour of Darkness that hasn't been released yet.