I don't think it's the volume/number of times you eat those foods, it's just that there's a chance of listeria every time you do. Just as there's a chance of listeria even if you're not pregnant. Problem is that listeria is very harmful to the foetus and can cause miscarriage.

The chance of listeria/salmonella is the same whether or not you're pregnant.

I'm sure someone will come in and explain that much better than I just have.