cancel, vb., makes canceled and canceling in AmE. Yet, in cancellation the -l- is doubled (-ll-) because the accent falls on the third syllable.
Can·ce(l)·la·tion Mid-16th c. Latin cancellat-, past participle of cancellare (cf. can·cel·(l)ing)
Longman Pronunciation dictionary reads
-ation ˈeɪʃən bears the primary word stress. In words of four or more syllables, a further rhythmic (secondary) stress falls two syllables further back (ˌconsoˈlation, conˌsideˈration, neˌgotiˈation, asˌsociˈation).
American English spelling an informal description reads:
The nineteenth-century formation parcellation can be seen as reflecting earlier English spelling, or it can be seen as a very conservative instance of British twinning, parallel to, say, cancellation, so rare that there has been no pressure to recognize a more regular variant spelling
I don't grasp how to generalize the spelling rule behind Garner's statement, so I'd appreciate any help.