The spelling of the word "faultier" is based on the pronunciation of the word, which is /ˈfɔltiɐ/. The first syllable "fault" is spelled with an "a" and not an "o" because it's pronounced with an "aw" sound. The second syllable "ier" is spelled with an "ie" and not an "ei" because it's pronounced with an "ee" sound. "Faultier" is a German word that means "sloth" in English. Its unique spelling and pronunciation make it an interesting word to learn and use in conversation.
* The statistics data for these misspellings percentages are collected from over 15,411,110 spell check sessions on www.spellchecker.net from Jan 2010 - Jun 2012.