This explains why the runs haven't been reduced much:
"Arun: "One team bowls out the other for 232 in 50 overs RR 4.64 and then it has to chase just 20 less in 40 overs at a runrate above 5. I will never understand Duckworth Lewis." It accounts for that fact that it is easier to bat 40 overs with ten wickets in hand as compared to 50 overs"
Source