These words were all originally spelled with two l's (in British English, which is why the English Oxford dictionary will not recognize the single-L spelling).

America has a long history of movements to simplify spelling and make it more phonetic. Webster was one of the first to publish Americanized spellings in his dictionary in the late 1800s (which is why you did find it in the Webster dictionary).

A committee for simplified spelling was created in the early 1900s in America, and they published the Handbook of Simplified Spelling to record these changes. One of the rules dictated that VERBS ending in double consonants, preceded by short vowels would drop their second consonant. Since cancellation is not a verb, it does not apply.

These words were all originally spelled with two l's (in British English, which is why the English Oxford dictionary will not recognize the single-L spelling).

Webster was one of the first to publish Americanized (more phonetic) spellings in his dictionary in the late 1800s (which is why you did find it in the Webster dictionary).

An American committee for simplified spelling published the Handbook of Simplified Spelling to record these changes in the early 1900s. One of the rules dictated that VERBS with double consonants, preceded by short vowels would drop their second consonant. Since cancellation is not a verb, the rule did not apply.