I'm not sure. One thing I can say from experience though is that bills in Japan are newer and cleaner than they are in the US. My impression is that it would be unthinkable in Japan to write on a bill with a pen or to crumple it up. Also, I'd guess bills that start to get worn are taken out of circulation much earlier.

Compare that to the US where I've seen a supermarket teller draw a line with a pen across a bill after accepting it (why?) or people pulling wadded cash balls out of their pockets to pay for a soda at a convenience store.

I don't know what accounts for the difference but I did like the typically crisp and new bills in Japan.