At Asda we were told they tend to do more checks when you are a newer user of the system and there is a bigger chance of being 'randomly' selected for checks if you keep removing things you have already scanned as you've decided you don't want items or change items for an alternative. I think we once swapped steak pies for chicken pies, plain Doritos for cheesy etc and put a couple of things back and got checked, the person who did the check then mentioned it might be because of taking things off the original scan list. I suppose people scan things, take them off and keep them in their trolley to avoid paying. They can then claim they must have been accidentally removed and were intending to pay for the items if they are found out as they had originally been scanned and it was a mistake they were removed. Hard for the shops to prove you were trying to steal stuff.
Tesco once mentioned getting a lot of multiple items can trigger a check too. When we use the big 24h Tesco we do a big shop, buying stuff to freeze too. As we regularly feed 6 people we tend to buy 2 family pies, 2 or 3 bags of potatoes, 2 bags of carrots, 2 jars of hunters chicken sauce, 4 jars of Korma sauce, 3 or 4 packs of chicken, 2 big boxes of mince, 3 x 6 packs of sausages (sometimes 6 packs to freeze half) etc. etc. etc. as a matter of course so adding more on for a big shop can multiply things up even more. A member of staff said this can trigger a check as it is easy to scan 2 items but have 3, scan 3 tins of soup but have 4 etc.
We scanned our last shopping at Asda ourselves and were checked. I did notice the chap who did the check scanned a few items we had 2 of, probably checking to make sure they had both been scanned, it would be very easy to put 2 items in and only scan one of them, therefore stealing a duplicate. So it looks like Asda check for multiple buying too.
A short while before Christmas last year a member of staff whose till we went through regularly and got to know enough to chat to about stuff other than shopping told us that someone had been arrested very recently for going through the scan and go tills. They had about 24 bottles of champagne in the trolley with something large draped over the top (an Asda that sells bedding etc.) to try to hide the bottles. They had scanned the stuff on top and only one bottle of champagne! Checks can be useful.