The British U

Sometimes I look at words and they appear to be spelled wrong. It usually happens with common words that I’ve seen and written over and over. Suddenly, something just looks off. I am not so much of an Anglophile that I read British printings of books, but that extra U that appears in certain words in the UK just seems like a better fit. For example, behaviour looks like the right word and behavior seems to be lacking. Spell check tells me otherwise.