This was the first time on a quiz that I had two left and got them with one answer! (I share your confusion about the discrepancy. Maybe a type-in was added after trusty was already more answered?)
"Fusty, musty, dusty" is the catchphrase in an Enid Blyton book I remember from my childhood, which gets repeated ad infinitum by the parrot Kiki in, I think, "The Castle of Adventure". I never thought it would come in useful.
You are right. They’re both not common enough for a quiz like this, but sawdusty is ten times more common according to Google Ngram. That’s why I gave Untrusty the type-in. I could just remove it, but I don’t want to reset the stats.
https://www.jetpunk.com/series/271570/words-that-end-in
Thanks! I like the little bit of added colour.