Learn English – “Inner” but not “outter”

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.

I think this has something to do with spellings such as

full, but hopeful, grateful and similar adjectives, and also fulfil BrE, AmE fulfill

till, but until

all, but almost, always and similar words.

Unfortunately I have never seen a spelling rule that covers the whole phenomenon and tries to give an explanation. It seems that two identical consonants are reduced to one consonant in compound words, just to reduce the effort of writing. But whether this is a general rule or a phenomenon found only in some compound words that is a problem I have never looked into more closely.

This is what I have found on the Internet

http://www.howtospell.co.uk/adding-all-till-full