I came here to say the exact same thing.

Absurdly common words like TIMESTAMP and SITEMAP aren't in this game's database, yet words that are obscure-to-me such as "beerier" and "sisal" are considered "common". There seems to be no rhyme nor reason to what makes a word common or not in this game. That said, both TIMESTAMP and SITEMAP deserve to be common words.

I'm flabbergasted by two things:

• "polypropylene" is considered a "common" word
  
• its root word "propylene" is considered "uncommon"

I doubt anyone without a background in chemistry knows either word. But I'm further confused as to how adding the "poly" prefix to an alleged "uncommon" word suddenly makes it "common".