Maybe I had something on the ground that I didn't notice. I kept my town perfectly clean. One day at about 1:00pm I temporarily set some stuff down in town to gather some other things. I figure it's safe until the next morning, so I save and quit. I came back a few hours later, talked to Isabelle, and she said the town was dirty.
Maybe it also checks when you talk with her? Maybe it checks when you load a game?
Also, does anyone have a Varia Suit? I am just missing the main part of the outfit and I am so weary from opening fortune cookies. Looking for King, Queen, and Knight chess pieces as well. I've seen them in my HHA, but they are customized and can't order.
My HHA is actually overflowing with homes. I get about 15 AC streetpasses a week. About half of them are very scrubby, with HHA scores of a couple thousand. There are a few with scores in the several hundred thousand. I don't know what they do to reach that. I actually saw one that was 1.1 million HHA points. I looked inside and didn't see anything special about it.