Not sure, but I am pretty sure it is genetic. It might have something to do with how tall their racks are, so eye guards may not serve a lot of purpose. It may have to do with "rubbing" trees, because most mule deer will rub brush.
I have killed some with 10" of eye guards and recently basically void of them. I think the older they get the more their eye guards develop, but some just genetically do not have much if any eye guards.
Now the hybrid (mule deer, whitetails) can get tricky. They will have whitetail features and their sheds can be tricky to determine if they are muley or whitetail.
Mule deer are a cool animal and I am slowly learning about them, I have three books on them and none of them go into detail about brow tines.