Listeria is a food poisoning bacterium that is Psychrotrophic  (It can still grow in cold temperatures)
The reason why the law for fridge temperatures and the recommended temperature are different is because it has been shown on growth models that Listeria grows nearly twice as fast at 8°c than it does at 5°c. So this shows that the recommended temperature of 5°c is to keep us and our customers safe from Listeria.

For more information visit the link below for the Food Standard Agency

http://www.food.gov.uk/news-updates/campaigns/germwatch/science-fsw/fridge-temperatures#.U9EWLO8g_IU