Words With Fluctuations
Saying that Words with Friends follows a different dictionary than Scrabble is tricky because Words with Friends doesn't really release a list of all the acceptable words in the lexicon. The Words with Friends site refers to the Enhanced North American Benchmark Lexicon (ENABLE), a public domain list, as its source, but it also admits to adding words like "texting" or "Zen" as it sees fit. Much like in Scrabble, proper nouns, abbreviations, prefixes, suffixes and words that need a hyphen or an apostrophe are no-nos. Unlike Scrabble, derogatory words or slurs are not allowed.
Many grumbles were heard about Scrabble allowing two particular words --"za" and "qi" -- in their game, while Words with Friends didn't. It appears that Words with Friends has now added those as acceptable. Another bad habit you might develop with Words with Friends: You can update the app, and more words might've been added. Those with older versions might have different words.
So it's best to familiarize yourself with Scrabble's acceptable word list (the Official Tournament and Club Word List) before plunging into a competitive game.