A question of definitons

I have both the Oxford dictionary and Webster's dictionary. When looking up new word I've always used the Oxford dictionary, leaving Webster's virtually untouched. Through some happenings today, an issue was brought up to my attention. The difference in definitions between the two dictionaries. For example, the word "facetious". The Oxford Dictionary goes as follows-



while the Merriam-Webster dictionary defines the word facetious like this-



Now the two definitions contain a similar idea, but Oxford definition states that the use of humor is inappropriate, the Webster dictionary has no such statements or implications. Also, the Oxford dictionary says "treating serious issues" while Webster's dictionary says "lacking serious intent" but that doesn't necessarily mean that the word means to treat serious issues with humor.

So one could envision the misunderstanding that may arise from this, if I were to use the Webster definition of the word I could say: "His article was largely facetious" of a person's completely trivial piece of literary work using the 2nd definition. Someone who only knows the Oxfordian (is that even a word?) definition would think that the person was making light of a serious issue though. Now this is just one example, I looked up a number of other words and found differences in the two dictionaries' definitions of them that may lead to the words being used a bit differently in conversation.

My question is: What is the meaning of this? My Webster's dictionary is from 1997, and my Oxford dictionary a more recent 2004 edition. Could a word "lose" an application in 7 years? Or is this caused by the Oxford dictionary being a British dictionary and Webster's being an American dictionary? Would the additional definition be acceptable in Britain?