session-time with incorrect calculations

Hi I am running FreeRadius 1.0.4 with Postgres. I have noticed that the sessiontime is sometimes calculated incorrectly in the radacct table. If you compare the acctsessionstart and the acctsessionend with the acctsessiontime it does not match. This is particularly true for connections ended with idle-timeout. When would session times generally be wrong and how can I stop this from happening ? Thanks Barry