Why does British English spell the American three letter word for derriere and add an "r" and?

add an "e" and take out one of the two "s"s behind the only a? Sorry to write so cryptically, but I'm trying to conform to the rules.
Update: Hey, how come Min can write ***?
Update 2: So spacing the letters works? Let's see....a s s
Update 3: No...not the same
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