-ize or -ise are often style matters and not spelling errors.

(http://oxforddictionaries.com/words/ize-ise-or-yse)

(http://blog.oxforddictionaries.com/2011/03/ize-or-ise/)

We’re English so it’s hard to balance the two; we used ‘favorite’ as that’s what they’re called on Twitter but proper English has crept in in places. We’ll probably move over to all American–English at some point.

Cheers! H