Now there has been many sides to why Blizzard did this. Some have been saying that it was the authors error when it was written in the War of the Ancients Trilogy. I don't by this for a second because Blizzard has always done a great job of making sure the details in the novels are what they should be.
The other theory is that models on the PTR are always a work in progress, and sometimes to get the content out for testing Blizzard puts in placeholder models or models that don't have all the fine details on them for when it goes live.
I actually believe this to be the case, because it has happened in the past quite a few times. For all those saying that no this is just Blizzard caving I am going to remind you all of Lore Marrowgar and his model journey on the PTR.