U just explained the meanings, he is talking about the actually look of the masks.. And they have a look to where u might get them mixed up or not be able to tell which is which.. But if u look at them together, u can see that they have different details.. Also i have heard people getting the wrong masks cause they get em mixed and go to artist who dont know japanese art..
well as far as I've understood there is more than one Oni, for example there is Ikazuchi and then there is Fudo, they are both Oni but different individual entities. Oni being an umbrella term as demon is in western culture. I've been doing research on japanese mythology and symbolism in for a couple years in preparation for my tattoos