That's because most people apparently don't know the difference. I've seen plenty of restored cars with the wrong grille and even more with the grille/headlight bezels painted incorrectly. Here's a 68/69 grille:
Here's a 70-72 grille:
As you can see, there's quite a bit of difference. Yes, either grill will FIT a 68-72, but only one will be right.