Yahoo, being a company based in the U.S. more than likely is using an American English dictionary for their spell check function. So it sees British spellings as being misspelled. I imagine the same would happen if an American using a British based site used American spellings for words. They would be seen as being spelled incorrectly.
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I guess because it is correct in the UK, but not the preferred spelling (behavior) in the US, so it is giving you the choice.
Yahoo, being a company based in the U.S. more than likely is using an American English dictionary for their spell check function. So it sees British spellings as being misspelled. I imagine the same would happen if an American using a British based site used American spellings for words. They would be seen as being spelled incorrectly.
A conflict between British English and American English? :S
Y!A has the strangest spell check i have ever run across
maybe they go by the queen's english dictionary
problematic situation. search from bing and yahoo. this will help!