I have been thinking about your $31 transaction in Lotte Myeongdong.

Did you use a US card, or HK card?

I bet this transaction happened on the floor B1 to 8, in the regular department store area, right? Because the one slip on the left is from the Duty Free, where you have to use your passport. That part takes USD only.

My experience with Lotte Myeongdong is that you do get a choice of DCC or not, but some sales guys just choose that for your without asking you. I've done purchases in the B1 supermarket, B1 specialty stalls, and regular stores on other floors, but never ran into a machine that is hardwired to choose USD.

Maybe yours was in 2012, and I went there in 2014. Something might have changed.