I can’t explain that, other than the influence of U.S. media and the utter confusion of many folks, worldwide, who simply want to write out their thoughts and have them understood without breaking a sweat because the spelling police are underfoot. 
I grew up in the U.S. and was educated in a U.S. public school system —certainly not the best schooling — but if spelling were a more widely respected and recognized talent, I could have ruled the world at age twelve.
Anyway, I grew up spelling r-e-c-o-g-n-i-z-e, o-r-g-a-n-i-z-e and p-a-t-r-o-n-i-z-e with ‘Z’. It never would have occurred to me to stick an ‘S’ in there.
When I was transplanted to Canada and subsequently worked as a journalist, I had to re-train my brain to substitute ‘S’ for ‘Z’. My editors specifically told me to use UK spellings, and I was particularly admonished in regards to my ‘S’ and ‘Z’ usage.
I then returned to the States, and consciously dropped the ‘S’ in favour of a ‘Z’ for my daughter’s homework. The ‘Z’ was correct.
(Back in Canada now, by the way.)
Having said all that, my somewhat outdated copy of the Globe & Mail Style Book shows the use of ‘Z’. At the same time, I am still fairly new to the Toronto area, and as my friends and colleagues back west had warned me, this place truly does seem like “practically the U.S.”, including the spelling used by the Toronto and so-called ‘national’ media.
Having muddied those waters for you, there’s this:
One must never rely solely on MS Word for grammar or spelling rules. MS Word is quite helpful, but often, I find, not necessarily correct.
One glaring example I find is the missing subjunctive mood, wherein “If I were a rich man” is mistakenly “corrected” by MS Word to read “If I was a rich man.”
But with my own copy of MS Word, U.S. English recognizes the ‘Z’ as correct; the U.K. English recognises the ‘S’ as correct.
What version are you using?
