It's a classic false-positive problem. Most times when the self-checkout clerk has to give you attention, the problem is stupidly innocuous, so they blindly approve, as they have been trained by the system that it isn't a real problem.

I'm sure plenty of things get by them this way.

My experience is that the assigned employee is always looking for something to do, because he can't leave the self-checkout area, but there isn't anything to actually do there. And well, the store better not accuse honest customers of anything, or else some stuff they really won't like will happen (and that applies to poor customers too).

Anyway, the experience is still so bad that I tend not to use it. But that's because the machines really suck.