I don't know... You live here for a while you start to pick up that spelling. Licence. Defence. Tyres. Rumour. That sort of thing. I get confused on FR sometimes because I try to stick with American spelling but some words (rumour, humour those ones especially) have really stuck in my head. Also, I can't even remember whether some words are spelled which way in which language and will never get them right anymore. Is it "organize" in the Queen's English and "organise" in American or vice versa?
Also, the article was posted in a Brit newspaper so I assume that any spelling editors would have changed his American spelling (if he used it) to the British version.