A single vs a double consonant issue.
Solution 1:
My German grammar says in compounds and derivations of one-syllable words ending with -ll such as
- all fill full skill till
one l is dropped in British English as in
- almost although to fulfil (AmE to fulfill) skilful fully until
ll remains before -ness: dullness fullness
Source: Adolf Lamprecht, Grammatik der englischen Sprache. Publisher Cornelsen. Page 343.