How to use CT figures for Chlorine

[Isaacson]

According to the WHO recommendations for drinking water safety http://www.who.int/water_sanitation_health/dwq/guidelines/en/ it says under residual chlorine that to achieve a log2 reduction in bacteria you need "Ct99 = 0.04–0.08 min·mg/l; 5 °C; pH 6-7)" By my understanding at 0.5mg/l (normal tap water) you would need less than 1 tenth of a minute contact (0.5 x 0.1 = 0.05). That can't be right, it would make tap water an effective disinfectant within minutes, can anyone explain why?