There are a lot of words in the english language that are like this, amoeba, ameba (yes, that is the current spelling in textbooks), etc. What happened here is that most of those words were originally greek, transliterated through latin. The Greeks had letters that Latin didn't, so the dipthongs were introduced. Since English is an ever-evolving language, people have decided that use of random extraneous letters is confusing and have thus removed them without changing the sound of the word. "Greek and Latin in Scientific Terminology" is a very good resource if you're interested in learning more. It is true, that the people living in the UK have generally avoided the changes that Americans made, though the changes, even in America are relatively recent, and many older scientists and doctors still used the dipthong-ed spellings.
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