I think they do more checks in the beginning, or if you missed to scan something as the system probably sees you as more or a “risc”. I can understand your frustration though

I know some stores mix self scanning and self checkout, usually they’re in the same exit area, or even at the same terminal

You people don’t have self scanning? Pick a barcode scanner at the entrance, scan everything when you put it in your bags in the cart, and just pay at the exit and walk out (unless you get a semi-rare random check). One of the favorite features of the store I use is that I fill the shopping list at home in the app (that can be shared with other accounts) and then I see the list in my phone or the store scanner, sorted by the order of sections in the store, so eg. all fruit and vegetables will be next on the list when I get to that section. I also like that you see your total in real time and the scanner reminds you if there’s a “3 for 2” or other offers.