Actually, that's really not true. There are even regional differences in the formulas used for these brand within the same country, and in particular two things that vary a lot are sweetness and degree of carbonation. Bottling plants are regional for these products, so it makes it easy to provide products that meet the preferences of the local populace.
I'm afraid your ability to discriminate these differences is perhaps a lot less effective than you think.
Anyway, the patent issue you bring up is an interesting one. I don't know the answer, and what happens if a company has a patent, but over a period of years modifies it's product.