(Damn, in the time it took me to type this, my favorite article got linked to already! Oh, well. Here it is anyway.)
There really is a bit more to it, at least historically. At the turn of the last century, tamale carts began serving “authentic” Mexican food on the streets of Houston and other Texas cities. They were anything but authentic. They were often tortilla wrapped (not corn meal) and served in a brown gravy which had cumin and chili powder added to it. (A brown gravy like salisbury steak gravy.) The idea was to claim authenticity, but serve food that was accessible to the Anglo palate.
In 1972, Diana Kennedy published a cookbook called The Cuisines of Mexico, where she first used the term “Tex-Mex” to distinguish this highly Anglicized food from the food made in Mexico. By 1973, restaurants were advertising more “authentic Mexican” food than ever, now including things like fajitas and chimichangas, which aren’t from Mexico either.
It’s fair to say that the majority of dishes we think of as Mexican, including foods now sold in tourist Mexico, aren’t. Not only fajitas and chimichangas, but soft (flour) tacos and burritos and margaritas (!) and even nachos and salsa. Sour cream is unheard of on your taco in Mexico.
On the other hand, there is some phenominal Mexican seafood, stews and things they do with small birds and rose petals that would make your hair curl and your Mama rise from her deathbed to dance in joy. These are rarely seen in the US, which is a real shame.