I wouldn't necessarily call that variation in consistency, but it's interesting you bring up the regional variations at McDonald's (Pizza Hut is one that has some /very/ interesting regional variations as well). This is something that can actually be jarring to Americans when they travel abroad, so much so that it's become something of a meme in travel communities for first-time travelers.

I think it's important to understand that it's pretty rare for a business to fail in its home market and succeed elsewhere, so winning in the home market first is important. A&W's home market and McDonald's home market is the US, and in the US McDonald's is /extremely/ consistent nationwide. It's probably the most consistent restaurant in any category.