In this answer, simplification is stated as one reason for spelling variations in American English. But unlike in color and favorite, the number of letters to spell the word in fulfil increases in ...
Possible Duplicate: When is “L” doubled? Is there any guidance on the usage of doubled consonants, particularly L, in British versus US English? For example 'Travelled' v. ...
I would tend to write double-l, but Google gives me more single-l, so I'm guessing it's an Atlantic divide thing. And I guess all the other *full words.