There are other sayings with differing usage as well. “Take this with a grain of salt” is meant as a warning to view the information taken with suspicion and to regard it as at best only partly true. At least it is meant this way by some, but some people use it slightly differently. To them, the information taken is either obscene, or something they know shouldn’t be said, usually gossip, that must be “salted” to make it “palatable”. It has little to do with the information’s accuracy. Even though the same phrase is being used, the different meaning allows us to view the two as different sayings with possibly different origins.
Even a saying such as “okay”, which to my knowledge is used the same way all over the world, has several different origin stories. One story holds that the word “okay” came from “OK”, which was an abbreviation of the business term “Oll Korrect”. Today, “OK” is used as an abbreviation for “okay”. So which came first? OK or okay?
Some origins of different sayings can be found here: http://list25.com/25-common-sayings-and-where-they-came-from/
The sometimes very different meanings ascribed to various sayings can be found here: http://www.inkdoodler.com/blog/observation-75