Check out the two screenshots crops above. Observe that in each, one instance of the word has been flagged as misspelled, and the other has not.
I ran into this problem while working on a translation job. The client had instructed me to overtype an existing Japanese document in order to preserve the formatting. After nosing around for a minute, I discovered that the upper line was marked as US English, and the lower line was marked as UK English (I prefer the UK spelling in this case). Not sure how those language settings got into a Japanese document.