But some are only spelled differently.

Many of these differences are in the suffixes of words otherwise the same:

European	American	example
-ise	-ize	Minimise, minimize	Also, there is a variation in the doubling of an "l" (L) at the end of a word, when adding a suffix — a variation that is bound by a rule....
-our	-or	Behaviour, behavior
-re	-er	Centre, center
-ce	-se	Defence, defense

Some academic/scientific words spelled with "ae" in European English use simply "e" in American:

European	American
*Anaesthesia	Anesthesia	* As for "aesthetic / esthetic," the usage is optional, in American — though the former seems the most common.
Archaeology	Archeology
Encyclopaedia	Encyclopedia