Interesting question :) Reading up on this has been a pretty crazy experience. My conclusions are the following:
- The reason for the change from fulfil to fulfill is attributed to inflection, with the spelling modified to suit the function of the word.
- The reason why it isn't spelt fullfill is similar to why parallel only retains the single 'l' at the end—to curb the apparent fugliness of an '-llell' cluster in words such as unparalleled etc.
- There is also a question of consistency involved.
But, unfortunately, another rule in the AE transition was to preserve the "base" word as much as possible, which is why skill/_skilful in BE became skill/_skillful_ in AE. However, this introduced a cluster of letters in words such as skillfully. To put it mildly, the result of exercise was and remains consistently inconsistent.
A related question on the difference in the AE/BE spellings of propelling covers similar territory. It also points to a WP page detailing the differences between American and British spelling which includes the following section on the madness of doubled consonants:
- The word parallel keeps a single -l- in British English, as in American English (paralleling, unparalleled), to avoid the unappealing
- Words with two vowels before a final l are also spelled with -ll- in British English before a suffix when the first vowel either acts as
a consonant (equalling and initialled; in the United States, equaling
or initialed), or belongs to a separate syllable (British fu•el•ling
and di•alled; American fu•el•ing and di•aled).
- British woollen is a further exception due to the double vowel (American: woolen). Also, wooly is accepted in American English,
though woolly prevails in both systems.
There's also a note on -ful and -fil:
In both American and British usages, words normally spelled -ll
usually drop the second l when used as prefixes or suffixes, for
example full→useful, handful; all→almighty, altogether; well→welfare,
welcome; chill→chilblain. The British fulfil and American fulfill are
never fullfill or fullfil. Johnson wavered on this issue. His
dictionary of 1755 lemmatises distil and instill, downhil and uphill.