The word "artier" is spelled with -ier at the end, which is a common suffix used to form comparative adjectives. This suffix is pronounced as /iə/ in IPA phonetic transcription. Therefore, when we say "artier", the "-ier" is pronounced as /iər/. The word "arty" is the base form of artier, and adding the suffix "-ier" means that it becomes "more arty" or "having more artistic qualities". It is important to remember the spelling and pronunciation of this suffix when using comparative adjectives in English.
* 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.