Unfortunately, security uses humans and humans are not infallible. The security staff are sat there looking at the screen for unspecified periods of time and they literally only have a few seconds to scan your bags image, so things will occasionally get through.
I am crew with a London based low cost airline and I was recently on a scheduled night stop. By mistake I had forgotten to remove my large toiletry bottles from my crew bag and I only realised when I got to my hotel down route. I chanced my luck the next door on returning to London on getting my large bottles back through to the UK - surprise surprise my large bottles were not picked up.
It might have something to do with the fact that I had taken out my regulation 100ml bottles and kept them in their 20cm x 20cm plastic bag which made security miss my larger bottles
